Westminster"  Series 


GI  ASS    MANUFACTURE 


GLASS 
MANUFACTURE 


BV 

WALTER    ROSENHAIN    B.A.   B.C.E. 

SUPERINTENDENT     OF     THE     DEPARTMENT     OF     METALLURGY     AND 

METALLURGICAL     CHEMISTRY    AT     THE     NATIONAL 

PHYSICAL     LABORATORY 


NEW    YORK 

D.    VAN    NOSTRAND    COMPANY 
23    MURRAY   AND    27    WARREN    STREETS 

1908 


GENERAL 


BRADBURY,    AGNEW,   &    CO.    LD.,   PRINTERS, 
LONDON    AND   TONBRIDGE. 


PREFACE 


THE  present  volume  on  Glass  Manufacture  has  been 
written  chiefly  for  the.  benefit  of  those  who  are  users  of 
glass,  and  therefore  makes  no  claim  to  be  an  adequate  guide 
or  help  to  those  engaged  in  glass  manufacture  itself.  For 
this  reason  the  account  of  manufacturing  processes  has 
been  kept  as  non-technical  as  possible ;  no  detailed  drawings 
of  plant  or  appliances  have  been  given,  and  only  a  few 
illustrative  diagrams  have  been  introduced  for  the  purpose 
of  avoiding  lengthy  verbal  descriptions.  In  describing  each 
process  the  object  in- view  has  been  to  give  an  insight  into 
the  rationale  of  each  step,  so  far  as  it  is  known  or  under- 
stood, and  thus  to  indicate  the  possibilities  and  limitations 
of  the  process  and  of  its  resulting  products  rather  than  to 
provide  a  detailed  guide  to  the  technique  of  the  various 
operations.  The  practical  aim  of  the  book  has  further  been 
safeguarded  by  the  fact  that  the  processes  described  in  these 
pages  are,  with  the  exception  of  those  described  as  obsolete, 
to  the  author's  definite  knowledge,  in  commercial  use  at  the 
present  time.  For  this  reason  many  apparently* ingenious 
and  beautiful  processes  described  in  earlier  books  on  glass 
have  not  been  mentioned  here,  since  the  author  could  find 
no  trace  of  their  employment  beyond  the  records  of  the 
various  patents  involved.  On  the  other  hand  the  reader 


207120 


vi  PREFACE. 

must  be  warned  to  bear  in  mind  that  the  peculiar  conditions 
of  the  glass  manufacturing  industry  have  led  to  the  practice 
on  the  part  of  manufacturers  of  keeping  their  processes  as 
secret  as  possible,  so  that  the  task  of  the  author  who  would 
give  an  accurate  account  of  the  best  modern  processes  used 
in  any  given  department  of  the  industry  is  beset  with  great 
difficulties.  The  author  has  endeavoured  to  steer  the  best 
course  open  to  him  under  these  circumstances,  and  he 
would  appeal  to  the  paucity  of  glass  literature  in  the 
English  language  as  evidence  of  the  difficulty  to  which  he 
refers. 

In  addition  to  these  difficulties,  which  arise  largely  from 
considerations  of  a  commercial  nature,  the  writer  of  a  book 
on  glass  is  further  confronted  with  technical  difficulties  of 
no  inconsiderable  order.  As  already  indicated,  the  aim  of 
the  present  author  has  been  to  describe  processes  from  the 
point  of  view  of  principles  and  methods  rather  than  as  mere 
rule-of-thumb  descriptions  of  manufacturing  manipulations, 
but  in  doing  this  he  is  met  at  every  turn  by  the  fact  that 
from  the  scientific  side  the  greater  part  of  the  field  of  glass 
manufacture  is  a  "  terra  incognita."  In  making  this 
statement  the  labours  of  many  eminent  scientific  workers 
are  by  no  means  forgotten,  but  the  entire  field  is  so  large 
and  beset  with  such  great  experimental  difficulties  that  even 
the  labours  of  a  list  of  investigators  that  includes  the 
names  of  Fraunhofer  and  Faraday,  Stokes,  Hopkinson, 
Abbe  and  Schott,  have  resulted  in  little  more  than  an 
accumulation  of  empirical  data  which,  while  they  have  been 
productive  of  great  direct  practical  results,  have  left  the 
science  of  glass  still  in  a  very  elementary  condition.  To 
take  two  examples  in  illustration  of  this  fact  we  may  mention 


PEEFACE.  vii 

the  question  of  the  connection  between  chemical  composition 
and  any  of  the  physical  properties  of  glass,  such  as  refrac- 
tion and  dispersion  of  light,  and  on  the  more  mechanical 
side  the  question  why  all  processes,  such  as  rolling  or 
moulding,  which  involve  the  contact  of  hot  glass  with  metal 
result  in  a  roughening  of  the  glass  surface.  The  former 
question  has  been  studied  by  several  of  the  investigators 
named  above,  Schott  and  Abbe  having  particularly  devoted 
an  enormous  amount  of  labour  and  money  to  the  study  of 
the  question  with  results  which  have  proved  disappointing 
from  the  scientific  point  of  view.  By  prolonged  experi- 
menting and  the  employment  of  a  costly  system  of  trial  and 
error  an  important  series  of  novel  and  useful  glasses  has 
been  produced  by  these  workers,  but  no  law  by  whose  aid 
the  optical  properties  of  a  glass  of  given  chemical  composi- 
tion could  be  predicted  has  yet  been  discovered,  and  as  a 
summary  of  the  known  facts  only  the  vaguest  general 
principles  are  available  for  the  guidance  of  those  who  wish 
to  produce  glasses  of  definite  properties.  The  same  applies 
in  a  similar  degree  to  most  of  the  other  properties  of  glass, 
with  the  exception,  perhaps,  of  density  and  thermal  expan- 
sion ;  attempts  to  generalise  from  the  known  data  of  a 
limited  number  of  glasses  generally  meet  with  unqualified 
failure.  The  conclusion  which  one  is  forced  to  admit  is 
that  the  fundamental  principles  underlying  the  nature  and 
constitution  of  glasses  have  yet  to  be  discovered.  A  study 
of  the  other  question  mentioned  above  as  an  example  of  the 
limitations  of  our  knowledge  leads  to  the  same  conclusion  ; 
an  almost  endless  succession  of  inventors  have  busied 
themselves  with  devices  for  overcoming  the  roughening 
action  of  rollers  and  moulds  upon  glass,  but  without  any 


viii  PEEFACE. 

real  success.  A  long  list  of  other  examples  of  the  same 
kind  could  be  given,  our  knowledge  of  the  physical  and 
chemical  principles  underlying  many  of  the  phenomena 
met  with  in  glass  manufacture  being  deplorably  deficient. 
It  will  thus  be  seen  that  to  write  a  truly  scientific  account 
of  glass  manufacture  is  at  the  present  time  impossible,  and 
the  reader  is  asked  to  bear  this  in  mind  if  he  should  find 
the  chemical  or  physical  explanations  given  in  this  book 
less  frequent  or  less  adequate  than  could  be  desired. 

Having  dwelt  somewhat  emphatically  on  the  limitations 
of  our  present  scientific  knowledge  as  applied  to  glass 
manufacture,  it  is  perhaps  scarcely  necessary  at  the  present 
time  to  emphasise  the  fact  that  this  state  of  affairs  should 
act  as  the  strongest  incentive  to  further  investigation  of  the 
whole  subject.  The  difficulty,  however,  lies  in  the  fact  that 
such  investigation  can  scarcely  be  carried  on  by  voluntary 
workers  in  ordinary  laboratories,  but  must  be  undertaken 
with  the  active  help  of  glass  manufacturers  at  their  works. 
Glass  is  essentially  a  substance  that  cannot  be  satisfactorily 
handled  in  small  quantities,  particularly  so  far  as  all  the 
phenomena  connected  with  its  production  and  manipulation 
while  hot  are  concerned;  the  influences  of  containing 
vessels,  of  furnace  gases  and  of  rapid  cooling  are  all 
enormously  exaggerated  if  ounces  instead  of  hundredweights 
or  tons  of  glass  are  used  for  experimental  purposes,  and 
these  influences  and  others  of  the  same  nature  vitally  affect 
all  the  results  of  small-scale  laboratory  operations.  The 
progress  of  our  scientific  knowledge  of  glass — and  the 
consequent  development  of  the  glass  industry  from  its 
present  state  where rule-of- thumb  and  "practical  experience" 
still  hold  excessive  sway — lies  in  the  hands  of  those 


PEEFACE.  ix 

concerned  in  the  industry  itself.  It  must  be  admitted  that  to 
undertake  such  work  involves  the  expenditure  of  much  time 
and  money  on  the  part  of  a  manufacturer,  while  the  field  is 
so  large  and  the  problems  so  complicated  that  any  adequate 
return  cannot  be  promised  for  the  immediate  future ;  on 
the  other  hand  the  very  size  of  the  field  and  the  difficulty 
of  the  problems  offers  the  promise  of  the  greatest  ultimate 
reward  ;  a  really  important  scientific  discovery  in  connection 
with  glass  would  be  certain  to  bring  in  its  train  industrial 
developments  whose  limits  it  is  impossible  to  foresee.  The 
industrial  success  of  the  glass-works  of  Schott  in  Jena  is 
often  quoted  as  a  brilliant  example  of  commercial  success 
resulting  from  purely  scientific  investigations  in  this  actual 
field ;  an  example  of  still  greater  magnitude  is  furnished  by 
the  success  of  the  aniline  dye  works  of  Germany  which  are 
built  up  on  purely  scientific  achievements.  The  glass 
industry  as  a  whole,  supplying  some  of  the  absolute 
necessaries  of  modern  life,  should  be  capable  of  offering  the 
greatest  rewards  to  success,  and  the  example  of  other 
industries  has  shown  that  ultimate  success  is  bound  to  reward 
properly-conducted  and  perseverant  scientific  research. 
Nowhere  is  this  more  urgently  needed  than  in  the  whole 
field  of  glass  manufacture. 

The  author  is  indebted  to  Mr.  W.  C.  Hancock  for  valuable 
assistance  in  the  reading  of  proofs  and  various  suggestions 
in  connection  with  the  contents  of  this  book. 


TABLE    OF    CONTENTS 


PAGE 

PREFACE  v 


CHAPTEE  I. 

THE  PHYSICAL  AND   CHEMICAL   PROPERTIES   OF   GLASS. 

Definition  of  the  term  "Glass" — Amorphous  structure  the  common 
feature  of  all  vitreous  bodies — Glass  a  congealed  fluid — Glasses 
not  definite  chemical  compounds  but  complex  solutions — Eange 
of  chemical  composition  available  for  glass-making — Considera- 
tions governing  chemical  composition — Influence  of  composition 
on  physical  properties — Chemical  stability  of  glass — Permanence 
of  glass  surfaces — Action  of  water,  acids,  and  alkalies  on  glass — 
Action  of  light  on  glass  .  .  .  .  .  .  .  p.  1 


CHAPTER   II. 

THE   PHYSICAL  PROPERTIES   OF   GLASS. 

Mechanical  properties  :  tensile  strength,  crushing  strength,  elasticity, 
ductility,  and  hardness — Thermal  properties  of  glass  :  thermal 
endurance,  coefficient  of  expansion,  thermal  conductivity — Ther- 
mometer glass — Electrical  properties  of  glass — Transparency  and 
colour  of  glass  .  .  .  .  .  .  .  .  .  p.  18 


xii  TABLE    OF   CONTENTS. 

CHAPTEE   III. 

THE   RAW   MATERIALS   OF   GLASS   MANUFACTURE. 

General  considerations  —  Chemical  purity,  moisture,  and  physical 
condition,  constancy  of  quality — Sources  of  silica,  sand  and 
sandstone — Felspar — Sources  of  alkali:  Soda  ash  (carbonate  of 
soda),  salt  cake  (sulphate  of  soda),  pearl  ash  (carbonate  of 
potash) — Alkali  nitrates — Natural  minerals  containing  alkalies — 
Sources  of  other  bases :  Lime,  chalk,  limestone,  slaked  lime — 
Gypsum  (sulphate  of  lime) — Barium  compounds — Magnesia  and 
zinc — Lead  oxide,  red  lead — Aluminium,  manganese,  arsenic — 
Carbon — Coke,  charcoal,  anthracite  coal  .  .  .  .  p.  35 

CHAPTER   IV. 

CRUCIBLES   AND   FURNACES   FOR   THE   FUSION   OF   GLASS. 

Fire-clay  and  silica-brick  —  Manufacture  of  glass-melting  pots — 
Drying  and  first  heating  of  pots — Blocks  for  tank  and  other 
furnaces — Uses  of  silica  brick — Furnaces — Coal-fired  and  gas- 
fired  furnaces — Gas  producers — Regenerative  furnaces,  principles 
and  construction  of  Siemens'  furnaces — Recuperative  furnaces — 
General  arrangements  of  modern  tank  furnaces  —  Relative 
advantages  of  tank  and  pot  furnaces  .  .  .  .  p.  54 

CHAPTER   V. 

THE   PROCESS   OF   FUSION. 

Mixing  of  raw  materials  by  hand  and  by  machinery — The  charging 
operation  —  Chemical  reactions  during  melting  of  carbonate 
mixtures,  and  of  sulphate  mixtures  —  Influence  of  carbon  on 
the  reactions — The  fining  process  .  .  .  .  .  p.  73 

CHAPTER   VI. 

PROCESSES   USED   IN   THE  WORKING   OF  GLASS. 

Ladling,  gathering,  and  casting  —  Limitations  of  ladling — Ladling 
used  for  rolled  glass,  gathering  for  blown  glass — Rolling  of  glass 
— Blowing  processes  and  operations — Use  of  moulds — Pressing — 
Moulding />.  84 


TABLE   OF   CONTEXTS.  xiii 

CHAPTEE    VII. 

BOTTLE   GLASS. 

Raw  materials — Furnaces — Predominance  of  tank  furnaces — Process 
of  blowing  bottles  by  hand — Gathering,  marvering,  blowing — 
Use  of  fire-clay  and  metal  moulds  —  Formation  of  neck  — 
Improved  appliances,  moulds  and  tools — Manufacture  of  bottles 
by  machinery  —  The  "Boucher"  bottle-blowing  machine  — 
Annealing  of  bottles — Large  bottles,  carboys — Aids  to  the  blower 
— Sievert's  process — Large  shallow  vessels,  bath-tubs  .  p.  95 

CHAPTEE  VIII. 

BLOWN  AND   PRESSED   GLASS. 

Eaw  materials  —  Bohemian  glass  and  flint  glass  —  Gathering  and 
blowing — Chair  work — Hand  work — Production  of  tumblers  by 
hand — Application  of  coloured  glass  to  blown  articles — Use  of 
moulds  as  aids  to  blowing — Eoughening  effect  of  moulds — Fire- 
polishing  by  reheating — Use  of  compressed  air — Pressed  glass — 
Moulds  and  presses  —  Capacity  and  limitations  of  pressing 
process  ..........  p.  108 

'*• 
CHAPTEE  IX. 

ROLLED  OR  PLATE  GLASS. 

Eolled  plate  glass — Furnaces — Eaw  materials — Process  of  ladling — 
The  rolling  table — Annealing — Cutting  and  sorting — Patterns  on 
rolled  plate — "  Figured"  rolled  plate — Machine  used  for  double- 
rolling — Polished  plate — Eaw  materials — Casting  from  melting 
pots— Special  casting  pots  —  The  rolling  table  —  Importance  of 
flatness — Annealing  kilns  —  Grinding  and  polishing  processes — 
Machines  used  for  grinding  and  polishing— Method  of  holding 
the  glass — Abrasives  and  polishing  materials — Theory  of  the 
polishing  process — Limiting  sizes  of  polished  plate — Homogeneity 
of  polished  plate — Uses  of  plate  glass — Bent  polished  plate — 
Mirrors — Bevelling,  process  and  machines — Wired  plate  glass, 
rolled  and  polished — Difficulties  and  limitations — Advantages  of 
wired  glass p.  122 


xiv  TABLE   OF   CONTENTS. 

CHAPTEE   X. 

SHEET  AND   CROWN   GLASS. 

Comparison  of  sheet  with  polished  plate — Raw  materials  for  sheet — 
Furnaces  :  various  forms  of  tank  furnaces — Blowing  process — 
Gathering,  forming  the  gathering  on  blocks,  forming  the  shoulder 
of  the  cylinder,  blowing  the  cylinder,  opening  the  end  of  the 
cylinder,  detaching  cylinder  from  pipe — Cutting  off  the  "cap" 
— Splitting  the  cylinder — Flattening  and  annealing — Cutting  and 
sorting  sheet-glass— Defects  of  sheet-glass — Variations  of  the 
process — Attempts  to  produce  "  sheet"  glass  by  rolling — Sie vert's 
process — Direct  drawing  processes — The  America.n  process  for 
drawing  cylinders — Fourcault's  processes — Difficulties  and  limita- 
tions— Crown  glass — The  blowing  process — Limitations  .  p.  149 


CHAPTER  XI. 

COLOURED     GLASSES. 

Definition  of  coloured  glass — Physical  causes  of  colour — Colouring 
substances :  copper,  silver,  gold,  carbon,  tin,  arsenic,  sulphur, 
chromium,  uranium,  fluorine,  manganese,  iron,  nickel,  cobalt — 
Range  and  depths  of  tints  available — Intensely  coloured  glasses 
— The  process  of  "flashing" — Character  of  "flashed"  glass- 
Colours  produced  on  glass  by  painting:  use  of  coloured  "glazes" 
as  paints — Ancient  stained  glass  and  modern  glass — Technical 
uses  of  coloured  glass,  photography,  railway  and  marine 
'.  .  .  178 


CHAPTER    XII. 

OPTICAL   GLASS. 

Nature  and  properties  of  optical  glass — Homogeneity — Formation 
and  removal  of  striae  in  solutions  and  in  glass — Transparency 
and  colour — Absorption  of  light  in  "decolourised"  glasses — 
Refraction  and  dispersion  —  Definitions — Refractive  index,  dis- 
persion, medium  dispersion,  the  quantity  v — Specification  of 
optical  properties  in  terms  of  certain  spectrum  lines — Table  of 


TABLE   OF   CONTENTS.  xv 

typical  optical  glasses  and  their  optical  constants  —  Crown  and 
flint  glasses— Eelation  between  refraction  and  dispersion  in  the 
older  and  newer  glasses — Work  of  Abbe  and  Schott — Applications 
of  the  new  glasses — Non-proportionality  of  dispersion  in  different 
types  of  glass — Resulting  imperfections  of  achromatism — -The 
relative  partial  dispersions  of  glasses — Pairs  of  glasses  giving 
perfect  achromatism  not  yet  fully  available  —  Constants  of 
Schott's  telescope  crown  and  flint  — -  Narrow  range  of  optical 
glasses,  consequent  limitations  in  lens  design — Causes  of  these 
narrow  limits  —  Possible  directions  of  extension  —  Chemical 
stability  of  optical  glasses  —  Double  refraction  in  optical  glass 
arising  from  imperfect  annealing p.  205 

CHAPTER    XIII. 

OPTICAL   GLASS. 

The  manufacture  of  optical  glass — Raw  materials — Mixing — Furnaces 
and  crucibles — Kilns  for  heating  pots — Transfer  of  pots  from  kiln 
to  melting  furnace — Introduction  of  cullet  and  raw  materials — 
The  fining  process,  difficulties  and  limitations  —  The  stirring 
process — The  final  cooling  of  the  glass — Rough  sorting  of  the 
glass  fragments — Moulding  and  final  annealing  of  the  moulded 
glass — Grinding  and  polishing  of  plates  and  discs  for  examina- 
tion ;  smallness  of  yield  obtained — Difficulty  of  obtaining  large 
blocks  of  perfect  glass p.  223 

CHAPTER  XIV. 

MISCELLANEOUS  PRODUCTS. 

Glass  tubing — Gathering  and  drawing  of  ordinary  tubes — Special 
varieties  of  tube — Combustion  tubes — Tubes  of  vitreous  silica — 
Varieties  of  vitreous  silica — Transparent,  glass -like  silica  ware — 
Great  cost  of  production  —  Translucent  "milky"  silica  ware 
produced  electrically — Great  thermal  endurance  of  vitreous  silica 
— Sensitiveness  to  chemical  action  of  all  basic  substances  at  high 
temperatures — Glass  rod  and  fibre — Glass  wool — Quartz  fibres — 
Glass  beads — Artificial  gems  —  Use  of  very  dense  flint  glass 
coloured  to  imitate  precious  stones — Means  of  distinguishing 


xvi  TABLE   OF   CONTENTS. 

imitations — Precious  stones  produced  by  artificial  means — Chilled 
glass — Great  strength  and  fragility  of  chilled  glass  —  Bupert's 
drops — Manufacture  of  "tempered"  glass  by  Siemens — De  La 
Bastie's  process — Massive  glass,  used  for  house  construction  and 
paving  blocks — Water-glass  (silicate  of  soda  or  potash),  manu- 
facture in  tank  furnaces — Glass  for  lighthouse  lenses  and  search- 
light mirrors — Production  by  casting  glass  in  iron  moulds — Sizes 
and  types  of  lenses  and  prisms  produced  .  .  .  .p.  238 


APPENDIX — Bibliography  of  Glass  Manufacture      .         .         .  p.  253 


GLASS     MANUFACTURE 


CHAPTEE  I. 

THE    PHYSICAL    AND    CHEMICAL    PROPERTIES    OF    GLASS. 

ALTHOUGH  the  term  "  glass  "  denotes  a  group  of  bodies 
which  possess  in  common  a  number  of  well-defined  and 
characteristic  properties,  it  is  difficult  to  frame  a  satisfactory 
definition  of  the  term  itself.  Thus  while  the  property  of 
transparency  is  at  once  suggested  by  the  word  "  glass," 
there  are  a  number  of  true  glasses  which  are  not  transparent, 
and  some  of  which  are  not  even  translucent.  Hardness  and 
brittleness  also  are  properties  more  or  less  characteristic  of 
glasses,  yet  very  wide  differences  are  to  be  found  in  this 
respect  also,  and  bodies,  both  harder  and  more  fragile  than 
glass,  are  to  be  found  among  minerals  and  metals.  Perhaps 
the  only  really  universal  property  of  glasses  is  that  of 
possessing  an  amorphous  structure,  so  that  vitreous  bodies 
as  a  whole  may  be  regarded  as  typical  of  "structureless" 
solids.  •  All  bodies,  whether  liquid  or  solid,  must  possess  an 
ultimate  structure,  be  it  atomic,  molecular  or  electronic  in 
character,  but  the  structure  here  referred  to  is  not  that  of 

G.M.  B 


2  GLASS  MANUFACTURE. 

individual  molecules  but  rather  the  manner  of  grouping  or 
aggregation  of  molecules. 

In  the  great  majority  of  mineral  or  inorganic  bodies  the 
molecules  in  the  solid  phase  are  arranged  in  a  definite 
grouping  and  the  body  is  said  to  have  a  crystalline  structure  ; 
evidences  of  this  structure  are  generally  visible  to  the 
unaided  eye  or  can  be  revealed  by  the  microscope.  Vitreous 
bodies  on  the  other  hand  are  characterised  by  the  entire 
absence  of  such  a  structure,  and  the  mechanical,  optical 
and  chemical  behaviour  of  such  bodies  is  consistent  only 
with  the  assumption  that  their  molecules  possess  the  same 
arrangement,  or  rather  lack  of  arrangement,  that  is  found 
in  liquids. 

The  intimate  resemblance  between  vitreous  bodies  and 
true  liquids  is  further  emphasised  when  it  is  realised 
that  true  liquids  can  in  many  instances  pass  into  the 
vitreous  state  without  undergoing  any  critical  change  or 
exhibiting  any  discontinuity  of  behaviour,  such  as  is 
exhibited  during  the  freezing  of  a  crystalline  body.  In  the 
latter  class  of  substances  the  passage  from  the  liquid  to  the 
crystalline  state  takes  place  at  one  definite  temperature,  and 
the  change  is  accompanied  by  a  considerable  evolution  of 
heat,  so  that  the  cooling  of  the  mass  is  temporarily  arrested. 
In  the  case  of  glasses,  on  the  other  hand,  the  passage  from 
the  liquid  to  the  apparently  solid  condition  is  gradual  and 
perfectly  continuous,  no  evolution  of  heat  or  retardation  of 
cooling  being  observed  even  by  the  aid  of  the  most  delicate 
instruments.  We  are  thus  justified  in  speaking  of  glasses 
as  "  congealed  liquids,"  the  process  of  congealing  in  this 
case  involving  no  change  of  structure,  no  re-arrangement  of 
the  molecules,  but  simply  implies  a  gradual  stiffening  of 


PHYSICAL  AND   CHEMICAL   PROPEKTIES  OF   GLASS.    3 

the  liquid  until  the  viscosity  becomes  so  great  that  the  body 
behaves  like  a  solid.  It  is,  however,  just  this  power  of 
becoming  exceedingly  stiff  or  viscous  when  cooled  down  to 
ordinary  temperatures  that  renders  the  existence  of  vitreous 
bodies  possible.  All  glasses  are  capable  of  undergoing  the 
change  to  the  crystalline  state  when  kept  for  a  sufficient 
time  at  a  suitable  temperature.  The  process  which  then 
takes  place  is  known  as  "  devitrification,"  and  sometimes 
gives  rise  to  serious  manufacturing  difficulties. 

Molten  glass  may  be  regarded  as  a  mutual  solution  of  a 
number  of  chemical  substances — usually  silicates  and 
borates.  When  cooled  in  the  'ordinary  way  these  bodies 
remain  mutually  dissolved,  and  ordinary  glass  is  thus 
simply  a  congealed  solution.  The  dissolved  substances 
have,  however,  natural  freezing-points  of  their  own,  and  if 
the  molten  mass  be  kept  for  any  length  of  time  at  a 
temperature  a  little  below  one  of  these  freezing-points,  that 
particular  substance  will  begin  to  solidify  separately  in  the 
form  of  crystals.  The  facility  with  which  this  will  occur 
depends  upon  the  properties  of  the  ingredients  and  upon 
the  proportions  in  which  they  are  present  in  the  glass.  In 
some  cases  this  devitrification  sets  in  so  readily  that  it  can 
scarcely  be  prevented  at  all,  while  in  other  cases  the  glass 
must  be  maintained  at  the  proper  temperature  for  hours 
before  crystallisation  can  be  induced  to  set  in.  In  either 
of  these  cases,  provided  that  the  glass  is  cooled  sufficiently 
rapidly  to  prevent  crystallisation,  the  sequence  of  events 
during  the  subsequent  cooling  of  the  mass  is  this  :  as  the 
temperature  falls  further  and  further  below7  the  natural 
freezing-point  of  one  or  other  of  the  dissolved  bodies,  the 
tendency  of  that  body  to  crystallise  out  at  first  rapidly 

B  2 


4  GLASS  MANUFACTURE. 

increases ;  as  the  temperature  falls,  however,  the  resistance 
which  the  liquid  presents  to  the  motion  of  the  molecules 
increases  at  a  still  greater  rate,  so  that  two  opposing  forces 
are  at  work,  one  of  them  an  increasing  tendency  towards 
crystallisation,  the  other  a  still  more  rapidly  increasing 
resistance  to  any  change.  There  is  thus  for  every  glass 
a  certain  critical  range  of  temperature  during  which  the 
greatest  tendency  exists  for  the  crystallising  forces  to  over- 
come the  internal  resistance  ;  through  this  range  the  glass 
must  be  cooled  at  a  relatively  rapid  rate  if  devitrification  is 
to  be  avoided ;  at  lower  temperatures  the  crystallising  forces 
require  increasingly  longer  periods  of  time  to  produce 
any  sensible  effect,  until,  as  the  ordinary  temperature  is 
approached,  the  forces  of  internal  resistance  entirely  prevent 
all  tendency  to  crystallisation. 

The  phenomena  just  described  in  reality  constitute  the 
natural  limit  to  the  range  of  bodies  which  can  be  obtained 
in  the  vitreous  state :  as  we  approach  this  limit  the  glass 
requires  more  and  more  rapid  cooling  through  the  critical 
range  of  temperature,  and  is  thus  more  and  more  liable  to 
devitrify  during  the  manufacturing  processes,  until  finally 
the  limit  is  set  when  no  industrially  feasible  rapidity  of 
cooling  suffices  to  retain  the  mass  in  the  vitreous  state. 

While  the  range  of  bodies  that  can  be  obtained  in  the 
vitreous  state  is  very  large,  only  a  comparatively  small 
number  of  substances  are  ordinarily  incorporated  in  indus- 
trial glasses.  With  the  exception  of  certain  special  glasses 
used  for  scientific  purposes,  such  as  the  construction  of  optical 
lenses,  thermometers  and  vessels  intended  to  resist  unusual 
treatment,  all  industrial  glasses  are  of  the  nature  of  mixed 
silicates  of  a  few  bases,  viz.,  the  alkalies,  sodium  and 


PHYSICAL   AND   CHEMICAL   PROPERTIES   OF   GLASS.     5 

potassium,  the  alkaline  earths,  calcium,  magnesium,  stron- 
tium, and  barium,  the  oxides  of  iron  and  aluminium 
(generally  present  in  minor  quantities),  and  lead  oxide. 
The  manner  in  which  these  various  elements  enter  into 
combination  and  solution  with  one  another  has  been  much 
investigated,  and  the  more  general  conclusions  have  been 
anticipated  in  what  has  been  said  above.  It  is  abundantly 
evident  that  glasses  are  not  definite  chemical  compounds, 
but  rather  solutions,  in  varying  proportions,  of  a  series  of 
definite  compounds  in  one  another.  In  many  cases  the 
actual  constitution  of  industrial  glasses  is  so  complex  as, 
for  the  present  at  all  events,  to  baffle  adequate  chemical 
expression. 

One  of  the  factors  that  limit  the  range  of  possible  com- 
positions of  glasses  has  already  been  indicated,  and  two 
others  must  now  be  discussed.  For  industrial  purposes, 
the  cost  and  rarity  of  the  ingredients  becomes  a  vital  bar  at 
a  certain  stage  ;  thus  the  use  of  such  elements  as  lithium, 
thallium,  etc.,  is  prohibitively  costly.  In  another  direction 
the  glass-maker  is  very  effectively  restrained  by  the 
limitations  of  his  furnaces  as  regards  temperature.  The 
presence  of  excessive  proportions  of  silica,  lime,  alumina, 
etc.,  tends  to  raise  the  temperature  required  for  the  free 
fusion  of  the  glass,  and  when  this  temperature  seriously 
exceeds  1600°  C.,  the  manufacture  of  the  glass  in  ordinary 
furnaces  becomes  impossible.  Thus  pure  silica  can  be 
converted  into  a  glass  possessing  very  valuable  properties, 
but  the  requisite  temperature  cannot  be  attained  in  regenera- 
tive gas-fired  furnaces  such  as  are  ordinarily  used  by  glass 
manufacturers..  The  production  of  this  glass  has  accordingly 
been  carried  on  upon  a  small  scale  only  by  means  of 


6  GLASS  MANUFACTURE. 

laboratory  furnaces  heated  by  oxy-acetylene  flames,  while 
latterly  a  less  perfect  variety  of  silica  glass-ware  has  been 
produced  on  a  large  scale  by  the  aid  of  electric  furnaces. 
Such  methods  are,  however,  obviously  limited  to  very 
special  products  commanding  special  prices. 

A  further  limitation  in  the  choice  of  chemical  components 
is  placed  upon  the  manufacturer  by  the  actual  chemical 
behaviour  of  the  glass  both  during  manufacture  and  in  use. 
As  regards  chemical  behaviour  during  manufacture,  it  must 
be  borne  in  mind  that,  although  glasses  are  of  the  nature 
of  solutions  rather  than  of  compounds,  yet  these  solutions 
tend  towards  a  state  of  saturation  ;  thus  a  glass  rich  in 
silica  and  deficient  in  bases  will  readily  dissolve  any  basic 
materials  with  which  it  may  come  in  contact,  while,  on  the 
other  hand,  a  glass  rich  in  bases  and  poor  in  acid  con- 
stituents such  as  silica,  boric  acid  or  alumina,  will  readily 
absorb  acid  bodies  from  its  surroundings.  During  the 
process  of  melting,  glass  is  universally  contained  in  fire- 
clay vessels.  These  are  chosen,  as  regards  their  own 
chemical  composition,  so  as  to  offer  to  the  molten  glass  a 
few  of  those  materials  in  which  the  glass  itself  is  deficient ; 
yet  a  limit  arises  in  this  respect  also,  since  glasses  very 
rich  in  bases,  such  as  the  very  dense  lead  and  barium  glass 
made  for  optical  purposes,  rapidly  attack  any  fire-clay  with 
which  they  may  come  in  contact.  The  finished  glass 
also  betrays  its  chemical  composition  by  its  chemical 
behaviour  towards  the  atmospheric  agents,  such  as  moisture 
and  carbonic  acid,  with  which  it  comes  in  contact;  glasses 
containing  an  excessive  proportion  of  alkali,  for  example, 
are  found  to  be  seriously  hygroscopic  and  to  undergo  rapid 
decomposition,  especially  in  a  damp  atmosphere. 


PHYSICAL  AND   CHEMICAL    PROPERTIES   OF   GLASS.     7 

Within  the  limits  set  by  these  considerations,  the  glass 
manufacturer  chooses  the  chemical  composition  of  his  glass 
according  to  the  purpose  for  which  it  is  intended  ;  for  most 
industrial  products  the  cheapest  and  most  accessible  raw 
materials  that  will  yield  a  glass  of  the  requisite  appearance 
are  employed,  while  for  special  purposes  the  dependence 
of  physical  properties  upon  chemical  composition  is  utilised, 
as  far  as  possible,  in  order  to  attain  a  glass  specially 
suited  to  the  particular  requirements  in  question.  Thus 
the  flint  and  barium  glasses  used  for  table  and  ornamental 
ware  derive  from  the  dense  and  strongly  refracting  oxides 
of  lead  and  barium  their  properties  of  brilliancy  and  weight. 
The  fusibility  and  softness  imparted  to  the  glass  by  the 
presence  of  these  bases  further  adapts  it  to  its  purpose  by 
facilitating  the  complicated  manipulations  to  which  the 
glass  must  be  subjected  in  the  manufacturing  processes. 

Taking  our  next  example  at  almost  the  opposite  extreme, 
the  hardest  "combustion  tubing,1'  which  is  intended  to 
resist  a  red  heat  without  appreciable  softening,  is  manufac- 
tured by  reducing  the  basic  contents  of  the  glass  to  the 
lowest  possible  degree,  especially  minimising  the  alkali 
content,  and  using  the  most  refractory  bases  available,  such 
as  lime,  magnesia,  and  alumina  in  the  highest  possible 
proportions.  Such  glass  is,  of  course,  difficult  to  melt,  and 
special  furnaces  are  required  for  its  production,  but  on  the 
other  hand  this  material  meets  requirements  which  ordinary 
soda-lime  or  flint  glass  tubing  could  never  approach. 
Another  instance  of  these  refractory  glasses  is  to  be  found 
in  the  Jena  special  thermometer  glasses  and  in  the  French 
(Tonnelot)  "  Verre  dur  " ;  the  best  of  these  glasses  show 
little  or  no  plasticity  at  temperatures  approaching  500°  C., 


8  GLASS  MANUFACTUEE. 

and  have  thus  rendered  possible  a  considerable  exten- 
sion of  the  range  of  the  mercury  thermometer.  Further 
modification  of  chemical  composition  has  resulted  in  the 
production  of  glasses  which  are  far  less  subject  to  those 
gradual  changes  which  occur  in  ordinary  glass  when  used 
for  the  manufacture  of  thermometers — changes  which 
vitiated  the  accuracy  of  most  early  thermometers.  A  still 
more  extensive  adaptation  of  chemical  composition  to  the 
attainment  of  desired  physical  properties  has  been  reached 
primarily  as  a  result  of  the  labours  of  Schott  and  Abbe,  in 
the  case  of  optical  glasses.  The  work  of  these  men,  and 
the  developments  which  have  followed  from  it,  both  at  the 
works  founded  by  them  at  Jena  and  elsewhere,  have  so 
profoundly  modified  our  knowledge  of  the  range  of  possi- 
bilities embraced  by  the  class  of  vitreous  bodies,  that  it  is 
not  at  all  easy  at  the  present  time  to  realise  the  former 
narrow  and  restricted  meaning  of  the  term  "glass."  The 
subject  of  the  dependence  of  the  optical  properties  of  glass 
upon  chemical  composition  will  be  referred  to  in  detail  in 
Chapter  XII.  on  "  Optical  Glass,"  but  the  outline  of  the 
influence  of  composition  on  properties  here  given  could  not 
be  closed  without  some  reference  to  this  pioneer  work  of 
the  German  investigators. 

The  chemical  behaviour  of  glass  surfaces,  to  which  we 
have  already  referred,  is  of  the  utmost  importance  to  all 
users  of  glass.  The  relatively  neutral  chemical  behaviour 
of  glass  is,  in  fact,  one  of  its  most  useful  properties,  and, 
next  to  its  transparency,  most  frequently  the  governing 
factor  in  its  employment  for  various  purposes.  Thus  the 
entire  use  of  glass  for-  table-ware  depends  primarily  upon 
the  fact  that  it  does  not  appreciably  affect  the  composition 


PHYSICAL  AND   CHEMICAL  PEOPERTIES  OF   GLASS.     9 

and  flavour  of  edible  solids  or  liquids  with  which  it  is 
brought  into  contact — a  property  which  is  only  very 
partially  shared  even  by  the  noble  metals.  Again,  the 
use  of  glass  windows  in  places  exposed  to  the  weather 
would  not  be  feasible  if  window-glass  were  appreciably 
attacked  by  the  action  of  water  or  of  the  gases  of  the 
atmosphere.  For  these  general  purposes,  it  is  true,  most 
ordinary  glasses  are  adequately  resistant,  but  this  degree 
of  perfection  in  this  respect  is  only  the  outcome  of  the 
centuries  of  experience  which  the  practical  glass-maker 
has  behind  him  in  the  manufacture  and  behaviour  of 
such  glass.  When,  however,  a  higher  degree  of  chemical 
resistance  is  required  for  special  purposes,  as  for  instance 
when  glass  is  called  upon  to  resist  exposure  to  hot,  damp 
climates,  or  is  intended  to  contain  corrosive  liquids,  the 
rules  which  are  an  adequate  guide  to  the  glass-maker  in 
meeting  ordinary  requirements  are  no  longer  sufficient, 
particularly  when  the  glass  is  expected  to  meet  other 
stringent  requirements  as  well.  It  has,  in  fact,  frequently 
happened  that  a  glass-maker,  in  striving  to  improve  the 
colour  or  quality  of  his  glass,  as  regards  freedom  from 
defects,  brilliancy  of  surface,  etc.,  has  spoilt  the  chemical 
durability  of  his  products.  The  reason  lies  in  the  fact, 
long  known  in  general  terms,  that  an  increased  alkali  con- 
tent reduces  the  chemical  resistance  of  glass,  while  at 
the  same  time  such  an  increase  of  alkali  is  the  readiest 
means  whereby  the  glass-maker  can  improve  his  glass  in 
other  respects  by  making  it  more  fusible  and  easier  to 
work  in  every  way. 

This  subject  of  the  chemical  stability  of  glass  surfaces 
attracted  much    attention    during  the    later  part  of    last 


10  GLASS  MANUFACTUBE. 

century,  and  careful  investigations  on  the  subject  were 
carried  out,  particularly  at  the  German  Keichsanstalt 
(Imperial  Physical  Laboratory)  at  Charlottenburg.  Here 
also  the  labours  of  Schott  and  Abbe  proved  helpful,  until 
at  the  present  time  such  glass  as  that  used  by  the  Jena 
firm  in  the  production  of  laboratory  ware,  and  certain 
other  special  glasses  of  that  kind,  are  fitted  to  meet  the 
most  stringent  requirements. 

Leaving  aside  the  inferior  glasses,  containing,  generally, 
more  than  15  per  cent,  of  alkali,  the  behaviour  of  glass 
surfaces  to  the  principal  chemical  agents  may  be  summed 
up  in  the  following  statements.  Pure  water  attacks  all 
glass  to  a  greater  or  lesser  extent ;  in  the  best  glasses  the 
prolonged  action  of  cold  water  merely  extracts  a  minute 
trace  of  alkalies,  but  in  less  perfect  kinds  the  extraction  of 
alkali  is  considerable  on  prolonged  exposure  even  in  the 
cold,  and  becomes  rapidly  more  serious  if  the  temperature 
is  raised.  Superheated  water,  i.e.,  water  under  steam 
pressure,  becomes  an  active  corroding  agent,  and  the  best 
glasses  can  only  resist  its  action  for  a  limited  time.  For 
the  gauge-glass  tubes  of  steam  boilers  working  at  the  high 
pressures,  which  are  customary  at  the  present  time,  specially 
durable  glasses  are  required  and  can  be  obtained,  although 
many  of  the  gauge-tubes  ordinarily  sold  are  quite  unfit  for 
the  purpose,  both  from  the  present  point  of  view  and  from 
that  of  strength  and  "  thermal  endurance." 

In  certain  classes  of  glass,  the  action  of  water,  especially 
when  hot,  is  not  entirely  confined  to  the  surface,  some 
water  penetrating  into  the  mass  of  the  glass  to  an  appreci- 
able depth.  The  exact  mechanism  of  this  action  is  not 
known,  but  the  writer  inclines  to  the  view  that  it  arises 


PHYSICAL  AND   CHEMICAL  PEOPEETIES  OF  GLASS.    11 

from  a  partial  hydration  of  some  of  the  silica  or  silicates 
present  in  the  glass.  If  such  glasses  be  dried  in  the 
ordinary  way  and  subsequently  heated,  the  surface  wfll  be 
riddled  with  minute  cracks,  some  glass  may  even  flake  off, 
and  the  whole  surface  will  be  dulled.  As  such  penetrating 
action  sometimes  takes  place — in  the  poorer  kinds  of  glass 
—by  the  action  of  atmospheric  moisture  when  the  glass  is 
merely  stored  in  a  damp  place,  it  is  often  mistaken  for 
"  devitrification."  This  latter  action,  however,  is  not  known 
to  occur  at  the  ordinary  temperature,  although  glass  when 
heated  in  a  flame  frequently  shows  the  phenomenon ;  it 
is,  however,  entirely  distinct  from  the  surface  "  corrosion  " 
just  described.  Water  containing  alkaline  substances  in 
solution  acts  upon  all  glasses  in  a  relatively  rapid  manner ; 
it  acts  by  first  abstracting  silica  from  the  glass,  the  alkali 
and  lime  being  dissolved  or  mechanically  removed  at  a 
later  stage.  Water  containing  acid  bodies  in  solution — 
i.e.,  dilute  acid — on  the  other  hand  acts  upon  most  varieties 
of  glass  decidedly  less  energetically  than  even  pure  water, 
and  much  less  vigorously  than  alkaline  solutions ;  this 
peculiar  behaviour  probably  depends  upon  the  tendency  of 
acids  to  prevent  the  hydration  of  silica,  this  substance 
being  thereby  enabled  to  act  as  a  barrier  to  the  solvent 
action  of  the  water  upon  the  alkaline  constituents  of  the 
glass.  The  better  varieties  of  glass  are  al«o  practically 
impervious  to  the  action  of  strong  acids,  although  certain 
of  these,  such  as  phosphoric  and  hydrofluoric,  exert  a 
rapid  action  on  all  kinds  of  glass.  Only  certain  special 
glasses,  containing  an  excessive  proportion  of  basic  con- 
stituents and  of  such  substances  as  boric  or  phosphoric 
acid,  are  capable  of  being  completely  decomposed  by  the 


12  GLASS  MANUFACTUEE. 

action  of  strong  acids,  such  as  hydrochloric  or  nitric,  the 
bases  entering  into  combination  with  the  acids,  while  the 
silicic  and  other  acids  are  liberated. 

In  connection  with  the  action  of  acids  upon  glass,  mention 
should  be  made  of  certain  special  actions  that  are  of  prac- 
tical importance.  The  dissolving  action  of  hydrofluoric 
acid  upon  glass  is,  of  course,  well  known.  It  is  used  in 
practice  both  in  the  liquid  and  gaseous  form,  and  also  in 
that  of  compounds  from  which  it  is  readily  liberated  (such 
as  ammonium  or  sodium  fluoride),  for  the  purpose  of 
"  etching "  glass,  and  also  in  decomposing  glass  for  pur- 
poses of  chemical  analysis.  Next  in  importance  ranks  the 
action  of  carbonic  acid  gas  upon  glass,  especially  in  the 
presence  of  moisture.  The  action  in  question  is  probably 
indirect  in  character ;  the  moisture  of  the  air,  condensing 
upon  the  surface  of  the  glass,  first  exerts  its  dissolving  action, 
and  thus  draws  from  the  glass  a  certain  quantity  of  alkali, 
which  almost  certainly  at  first  goes  into  solution  as  alkali 
hydrate  (potassium  or  sodium  hydroxide)  ;  this  alkaline 
solution,  however,  rapidly  absorbs  carbonic  acid  from  the 
air,  anci.the  carbonate  of  the  alkali  is  formed.  If  the  glass 
dries,  this  carbonate  forms  a  coating  of  minute  crystals  on 
the  surface  of  the  glass,  giving  it  a  dull,  dimmed  appear- 
ance ;  this,  however,  only  occurs  ordinarily  with  soda 
glasses,  since  the  carbonate  of  potassium  is  too  hygroscopic 
to  remain  in  the  dry  solid  state  in  any  ordinary  atmos- 
phere. Potash  glasses  are,  as  such,  no  more  stable  chemi- 
cally than  soda  glasses,  but  they  are  for  the  reason  just 
given  less  liable  to  exhibit  a  dim  surface.  If  the  dimming 
process,  in  the  case  of  a  soda  glass,  has  not  gone  too  far, 
the  brightness  of  the  surface  of  the  glass  may  be  practically 


PHYSICAL  AND   CHEMICAL  PROPEKTIES  OF  GLASS.    13 

restored  by  washing  it  with  water,  in  which  the  minute 
crystals  of  carbonate  of  soda  readily  dissolve,  while  separated 
silica  is  removed  mechanically.  An  attempt  made  to  clean 
the  same  dimmed  surface  by  dry  wiping  would  only  result 
in  finally  ruining  the  surface,  since  the  small  sharp  crystals 
of  carbonate  of  soda  would  be  rubbed  about  ovrr  the 
surface,  scratching  it  in  all  directions. 

The  dimming  process  in  the  case  of  the  less  resistant 
glasses  is  not  only  confined  to  the  formation  of  alkaline 
carbonates ;  the  films  of  alkaline  solution  which  are  formed 
on  the  surface  of  glass  form  a  ready  breeding-ground  for 
certain  forms  of  bacteria  and  fungi,  whose  growth  occurs 
partly  at  the  expense  of  the  glass  itself ;  the  precise  nature 
of  these  actions  has  not  been  fully  studied,  but  there  can 
be  little  doubt  that  silicate  minerals — and  glass  is  to  be 
reckoned  among  these — are  subject  to  bacterial  decom- 
position, a  well-known  example  in  another  direction  being 
the  "  maturing "  of  clays  by  storage  in  the  dark,  the 
change  in  the  clay  being  accompanied  by  an  evolution  of 
ammonia  gas.  In  the  case  of  glass  it  has  been  shown  that 
specks  of  organic  dust  falling  upon  a  surface  give  rise  to 
local  decomposition.  In  this  connection  it  is  interesting 
to  note  the  effect  of  the  presence  of  a  small  proportion 
of  boric  acid  in  some  glasses.  The  presence  of  this 
ingredient  in  small  proportions  is  known  to  render  the 
glass  more  resistant  to  atmospheric  agencies,  and  more 
especially  to  render  it  less  sensitive  to  the  effects  of 
organic  dust  particles  lying  upon  the  surface.  It 
has  been  suggested  —  probably  rightly — that  the  boric 
acid,  entering  into  solution  in  the  film  of  surface 
moisture,  exerts  its  well-known  antiseptic  properties, 


14  GLASS  MANUFACTURE. 

thus    protecting    the    glass    from   bacterial   and    fungoid 
activity. 

The  durability  of  glass  under  the  action  of  atmospheric 
agents  is  a  matter  of  such  importance  that  numerous 
efforts  have  been  made  to  establish  a  satisfactory  test 
whereby  this  property  of  a  given  glass  may  be  ascertained 
without  actually  awaiting  the  results  of  experience  obtained 
by  actual  use  under  unfavourable  conditions.  One  of  the 
earliest  of  the  tests  proposed  consisted  in  exposing  surfaces 
of  the  glass  to  the  vapour  of  hydrochloric  acid.  For  this 
purpose  some  strong  hydrochloric  acid  is  placed  in  a  glass 
or  porcelain  basin,  and  strips  of  the  glass  to  be  tested  are 
placed  across  the  top  of  the  basin,  the  whole  being  covered 
with  a  bell-jar.  After  several  days  the  glass  is  examined, 
and  as  a  rule  the  le&&  stable  glasses  show  a  dull,  dimmed 
surface  as  compared  with  the  more  stable  ones.  A  more 
satisfactory  form  of  test  depends  upon  the  fact  that 
aqueous  ether  solutions  react  readily  with  the  less  stable 
kinds  of  glass ;  if  a  suitable  dye,  such  as  iod-eosin,  be  dis- 
solved in  the  water-ether  solution,  then  the  effect  upon  the 
less  stable  glasses  when  immersed  in  the  solution  is  the 
formation  of  a  strongly  adherent  pink  film.  The  density 
or  depth  of  colour  of  this  film  may  be  regarded  as  measur- 
ing the  stability  of  the  glass ;  the  best  kinds  of  glass 
remain  practically  free  from  coloured  film  even  on  pro- 
longed exposure.  A  test  of  a  somewhat  different  kind  is 
one  devised  in  its  original  form  by  Dr.  Zschimmer,  of  the 
Jena  glass  works  ;  this  depends  upon  the  fact  that  the  dis- 
integrating action  of  moist  air  can  be  very  much  accelerated 
if  both  the  moisture  and  the  temperature  of  the  air  sur- 
rounding the  glass  be  considerably  increased.  For  this 


PHYSICAL  AND   CHEMICAL  PBOPEKT1ES  OF  GLASS.    15 

purpose  the  samples  of  glass  are  exposed  to  a  current  of 
air  saturated  with  moisture  at  a  temperature  of  about 
80°  C.  in  a  specially  arranged  incubator  for  one  or  more 
days,  means  being  provided  for  securing  a  constant  stream 
of  moist  air  during  the  whole  time.  On  examining  the 
glass  surfaces  after  this  exposure— any  wiping  or  other 
cleaning  of  the  surfaces  being  avoided — various  qualities  of 
glass  are  found  to  show  widely  varying  appearances.  The 
best  and  most  stable  glasses  remain  entirely  unaffected ; 
less  stable  kinds  show  small  specks,  which  merge  into  a 
generally  dulled .  surface  in  unstable  kinds.  There  is 
no  doubt  that  this  test  gives  a  sharp  classification  of 
glasses,  but  it  yet  remains  to  be  proved  that  this  classi- 
fication agrees  with  their  true  relative  durability  in 
practice ;  the  writer  is  inclined  to  doubt  whether  this 
is  really  the  case,  since  certain  glasses  that  have  proved 
very  satisfactory  in  this  respect  in  practical  use  all  over 
the  world  were  classed  among  the  less  stable  kinds  by 
this  test. 

Before  leaving  the  subject  of  the  chemical  behaviour  of 
glass,  a  reference  should  be  made  to  the  changes  which  glass 
undergoes  when  acted  upon  by  light  and  other  radiations. 
Under  the  influence  of  prolonged  exposure  to  strong  light, 
particularly  to  sunlight,  and  still  more  so  to  ultra-violet 
light,  or  the  light  of  the  sun  at  high  altitudes,  practically 
all  kinds  of  glass  undergo  changes  which  generally  take  the 
form  of  changes  of  colour.  Glasses  containing  manganese 
especially  are  apt  to  assume  a  purple  or  brown  tinge  under 
such  circumstances,  although  the  powerful  action  of  radium 
radiations  is  capable  of  producing  similar  discoloration  in 
glasses  free  from  manganese.  Apart  from  these  latter 


16  GLASS  MANUFACTUBE. 

effects,  of  which  very  little  is  known  as  yet,  there  can  be  no 
doubt  that  the  action  of  light  brings  about  chemical  changes 
within  the  glass,  but  it  is  by  no  means  easy  to  ascertain 
the  true  nature  of  these  changes,  although  they  most  pro- 
bably consist  in  a  transfer  of  oxygen  from  one  to  another 
of  the  oxides  present  in  the  glass.  Although  it  has  not 
been  definitely  proved,  it  seems  very  unlikely  that  the  glass 
either  loses  or  gains  in  any  constituent  during  these 
changes.  Good  examples  of  the  changes  undergone  by 
glass  under  the  action  of  sunlight  are  frequently  found  in 
skylights,  where  the  oldest  panes  sometimes  show  a  decided 
purple  tint  which  they  did  not  possess  when  first  put  in 
place.  The  glass  spheres  of  the  instruments  used  for 
obtaining  records  of  the  duration  of  sunshine  at  meteoro- 
logical stations  also  show  signs  of  the  changes  due  to  light 
—the  glass  of  these  spheres  when  new  has  a  light  greenish 
tint,  but  after  prolonged  use  the  colour  changes  to  a 
decided  yellow.  The  coloured  glass  in  stained-glass 
windows  also  shows  signs  of  having  undergone  changes  of 
tint  in  consequence  of  prolonged  exposure  to  light ;  glass 
removed  from  ancient  windows  usually  shows  a  deeper  tint 
in  those  portions  which  have  been  protected  from  the 
direct  action  of  light  by  the  leading  in  which  the  glass  was 
set,  and  it  is  at  least  an  open  question  whether  the  beauty 
of  ancient  glass  may  not  be,  in  part,  due  to  the  mellowing 
effect  of  light  upon  some  of  the  tints  of  the  design.  This 
photo-sensitiveness  of  glass  is  also  of  some  importance  in 
connection  with  the  manufacture  of  photographic  plates. 
It  has  been  found  that  if  the  glass  plate  of  a  strongly- 
developed  negative  be  cleaned,  a  decided  trace  of  the  former 
image  is  retained  by  the  glass,  and  this  image  is  apt  to  re- 


PHYSICAL  AND   CHEMICAL   PEOPERTIES   OF   GLASS.    17 

appear  as  a  "  ghost  "  if  the  same  glass  be  again  coated  with 
sensitive  emulsion  and  again  exposed  and  developed.  The 
best  makers  of  plates  recognise  this  fact  and  do  not  re-coat 
glass  that  has  once  been  used  for  the  production  of  a 
negative. 


G.M. 


CHAPTEE  II. 

THE    PHYSICAL    PROPERTIES    OF    GLASS. 

The  Mechanical  Properties  of  Glass  are  of  considerable 
importance  in  many  directions.  Although  glass  is  rarely 
used  in  such  a  manner  that  it  is  directly  called  upon  to 
sustain  serious  mechanical  stresses,  the  ordinary  uses  of 
glass  in  the  glazing  of  large  windows  and  skylights  depend 
upon  the  strength  of  the  material  to  a  very  considerable 
extent.  Thus  in  the  handling  of  plate-glass  in  the  largest 
sheets,  the  mechanical  strength  of  the  plates  must  be  relied 
upon  to  a  considerable  extent,  and  it  is  this  factor  which 
really  limits  the  size  of  plate  that  can  be  safely  handled 
and  installed.  The  same  limitation  applies  to  sheet-glass 
also,  for,  although  its  lighter  weight  renders  it  less  liable 
to  break  under  its  own  weight,  its  thinner  section  renders 
it  much  more  liable  to  accidental  fracture.  In  special 
cases,  also,  the  mechanical  strength  of  glass  must  be  relied 
upon  to  a  considerable  -  extent.  Gauge  tubes  of  high- 
pressure  boilers,  port-hole  glasses  in  ships,  the  glass  prisms 
inserted  in  pavement  lights,  and  the  glass  bricks  which 
have  found  some  use  in  France,  as  well  as  champagne 
bottles  and  mineral  water  bottles  and  syphons,  are  all 
examples  of  uses  in  which  glass  is  exposed  to  direct 
stresses.  It  is,  therefore,  a  little  surprising  that  while  the 


THE   PHYSICAL   PKOPEKTIES   OF   GLASS.  19 

mechanical  properties  of  metals,  timbers,  and  all  manner 
of  other  materials  have  been  studied  in  the  fullest  possible 
manner,  those  of  glass  have  received  very  little  attention, 
at  all  events  so  far  as  published  data  go.  One  reason  for 
this  state  of  affairs  is  probably  to  be  found  in  the  fact  that 
it  is  by  no  means  easy  to  determine  the  strength  of  so 
brittle  and  hard  a  body  as  glass.  As  a  consequence  even 
the  scanty  data  available  can  only  be  regarded  as  first 
approximations.  The  following  data  are  only  intended  to 
give  an  idea  of  the  general  order  of  strength  to  be  looked 
for  in  glass : — 

Tensile  strength : 

From  1  to  4  tons  per  sq.  in.  (Trautwine). 

„      J  to  1J  „          „       „  (Henrivaux). 

.,      2  to  5j  „          ,,       „  (WinkelmannandSchott). 

„      5  to  6     „          „       „  (Kowalski). 

Crushing  strength : 

From  9  to  16  tons  per  sq.  in.  (Trautwine). 

„      3  to  8      ,,          „       „    (WinkelmannandSchott). 
„      20  to  27  „          „       „    (Kowalski). 

Of  the  above  figures  the  experiments  of  Winkelmann 
and  Schott  are  probably  by  far  the  most  reliable,  but  these 
refer  to  a  series  of  special  Jena  glasses,  selected  with  a 
view  to  determining  the  influence  of  chemical  composition 
on  mechanical  properties,  and,  unfortunately,  this  series 
does  not  include  glasses  at  all  closely  resembling  those 
ordinarily  used  for  practical  purposes.  The  attempt  to 
connect  tensile  and  crushing  strength  with  chemical 
composition  was  also  only  very  partially  successful;  but 

c  2 


20  GLASS  MANUFACTUEE. 

the  results  serve  to  show  that  the  chemical  composition 
has  a  profound  influence  on  the  mechanical  strength  of 
glass,  so  that  hy  systematic  research  it  would  probably  be 
possible  to  produce  glasses  of  considerably  greater  mechanical 
strength  than  those  at  present  known.  It  must  be  noted 
in  this  connection  that  the  mechanical  properties  of  glass 
depend  to  a  very  considerable  extent  upon  the  rate  of 
cooling  which  the  specimen  in  question  has  undergone.  It 
is  well  known  that  by  rapid  cooling,  or  quenching,  the 
hardness  of  glass  can  be  considerably  increased ;  such 
treatment  also  increases  the  strength  both  as  against 
tension  and  compression,  and  numerous  processes  have 
been  put  forward  for  the  purpose  of  utilising  these  effects 
in  practice.  Unfortunately  the  "  hardened  "  glass  thus 
obtained  is  extremely  sensitive  to  minute  scratches,  and 
flies  to  pieces  as  soon  as  the  surface  is  broken,  and 
the  great  internal  stress  which  always  exists  in  such  glass 
is  thereby  relieved.  All  these  peculiarities  are,  of  course, 
dependent  as  to  their  degree  upon  the  rapidity  with  which 
the  glass  has  been  cooled,  and  the  aim  of  inventors  in  this 
field  has  been  to  devise  a  rapid  cooling  process  which 
should  strike  the  happy  mean  between  the  increased 
strength  and  the  induced  brittleness  resulting  from 
quenching.  Thus  processes  for  "tempering"  glass  by 
cooling  it  in  a  blast  of  steam  or  in  a  bath  of  hot  oil  or 
grease  have  been  brought  forward ;  but,  although  some 
such  glass  is  manufactured,  no  very  extensive  practical 
application  has  resulted. 

Elasticity  and  Ductility  of  Glass. — In  a  series  of  glasses 
investigated  by  Winkelmann  and  Schott,  the  modulus  of 
elasticity  (Young's  Modulus)  varied  from  3,500  to  5,100 


THE   PHYSICAL    PROPERTIES   OF   GLASS.  21 

tons  per  sq.  in.,  the  value  being  largely  dependent  upon 
the  chemical  composition  of  the  glass.  Measurable  ductility 
has  not  been  observed  in  glass  under  ordinary  conditions 
except  in  the  case  of  champagne  bottles  under  test  by 
internal  hydraulic  pressure  ;  in  these  tests  it  was  found 
that  a  permanent  increase  of  volume  of  a  few  tenths  of  a 
cubic  centimetre  could  be  obtained  by  the  application  of 
an  internal  pressure  just  short  of  that  required  to  burst 
the  bottle — pressure  of  the  order  of  18  to  30  atmospheres 
being  involved.  This  small  permanent  set  has  been 
ascribed  to  incipient  fissuring  of  the  glass,  and  this 
explanation  is  probably  correct.  On  the  other  hand,  it  is 
in  the  writer's  opinion  very  probable  that  glass  is  capable 
of  decided  flow  under  the  prolonged  action  of  relatively 
small  forces  ;  the  behaviour  of  large  discs  of  worked  optical 
glass  suggests  some  such  action,  but  the  view  as  yet  lacks 
full  experimental  confirmation. 

The  Hardness  of  glass  is  a  property  of  some  importance 
in  most  of  the  applications  of  glass.  The  durability  of 
glass  objects  which  are  exposed  to  handling  or  to  periodical 
cleaning  must  largely  depend  upon  the  power  of  the  glass 
to  resist  scratching ;  this  applies  to  such  objects  as  plate- 
glass  windows  and  mirrors,  spectacle  and  other  lenses,  and 
in  a  minor  degree  to  table-ware.  On  the  other  hand,  the 
exact  definition  and  means  of  measuring  hardness  are  not 
yet  satisfactorily  settled.  Experimenters  have  found  it 
very  difficult  to  measure  the  direct  resistance  to  scratching, 
since  it  is  found,  for  example,  that  two  glasses  of  very 
different  hardness  are  yet  capable  of  decidedly  scratching 
each  other  under  suitable  conditions.  Resort  has  therefore 
been  had  to  other  methods  of  measuring  hardness ;  the 


22  GLASS  MANUFACTUBE. 

method  which,  from  the  experimental  point  of  view,  is, 
perhaps,  the  most  satisfactory,  depends  upon  principles 
laid  down  by  Hertz  and  elaborated  experimentally  by 
Auerbach.  This  depends  upon  measuring  the  size  of  the 
circular  area  of  contact  produced  when  a  spherical  lens  is 
pressed  against  a  flat  plate  of  the  same  glass  with  a  known 
pressure.  Auerbach  himself  found  some  difficulty  in 
deciding  the  exact  connection  between  the  "indentation 
modulus  "  thus  determined  and  the  actual  hardness  of  the 
glass.  This  method  is,  therefore,  of  theoretical  interest 
rather  than  of  use  in  testing  glasses  for  hardness.  A  test 
of  a  more  practical  kind  consists  in  exposing  specimens  of 
the  glasses  to  be  tested  to  abrasion  against  a  revolving  disc 
of  cast-iron  fed  with  emery  or  other  abrasive,  and  to 
measure  the  loss  of  weight  which  results  from  a  given 
amount  of  abrading  action  under  a  known  contact  pressure. 
If  a  number  of  specimens  of  different  glasses  are  exposed 
to  this  test  at  one  time,  a  very  good  comparison  of  their 
power  of  resisting  abrasion  can  be  obtained.  It  is  not 
quite  certain  that  this  test  measures  the  actual  "hardness" 
of  the  glass,  but  it  affords  some  information  as  to  its  power 
of  resisting  abrasion,  and  for  many  purposes  this  power  is 
the  important  factor. 

Hardness  being,  as  indicated  above,  a  somewhat  indefinite 
term,  it  is  not  possible  to  give  any  precise  statement  as  to 
the  influence  of  chemical  composition  upon  the  hardness 
of  glass.  In  general  terms  it  may  be  said  that  glasses  rich 
in  silica  and  lime  will  be  found  to  be  hard,  while  glasses 
rich  in  alkali,  lead  or  barium,  are  likely  to  be  soft.  It 
must,  however,  be  borne  in  mind  that  rapid  cooling,  or 
even  the  lack  of  careful  annealing,  will  produce  a  very 


THE  PHYSICAL  PROPERTIES   OF   GLASS.  23 

great  increase  of  hardness  in  even  the  softest  glasses.  The 
actual  behaviour  of  a  given  specimen  of  glass  will,  there- 
fore, depend  at  least  as  much  upon  the  nature  of  the 
processes  which  it  has  undergone  as  upon  its  chemical 
composition. 

The  Thermal  Properties  of  Glass,  although  not  of  such 
general  importance  as  the  mechanical  properties,  are  yet 
of  considerable  interest  in  a  large  number  of  the  practical 
uses  to  which  glass  is  constantly  applied.  Perhaps  the 
most  important  of  these  properties  is  that  known  as 
thermal  endurance,  which  measures  the  amount  of  sudden 
heating  or  cooling  to  which  glass  may  be  exposed  without 
risk  of  fracture;  the  chimneys  employed  in  connection 
with  incandescent  gas  burners,  boiler  gauge  glasses, 
laboratory  vessels,  and  even  table  and  domestic  utensils 
are  all  exposed  at  times  to  sudden  changes  of  temperature, 
and  in  many  cases  the  value  of  the  glass  in  question 
depends  principally  upon  its  power  of  undergoing  such 
treatment  without  breakage.  The  property  of  "  thermal 
endurance  "  itself  depends  upon  a  considerable  number  of 
more  or  less  independent  factors,  and  their  influence  will 
be  readily  understood  if  we  follow  the  manner  in  which 
sudden  change  of  temperature  produces  stress  and,  some- 
times, fracture  in  glass  objects.  If  we  suppose  a  hot  liquid 
to  be  poured  into  a  cold  vessel,  the  first  effect  upon  the 
material  of  the  vessel  will  be  to  raise  the  temperature  of 
the  inner  surface.  Under  the  influence  of  this  rise  of 
temperature  the  material  of  this  inner  layer  expands,  or 
endeavours  to  expand,  being  restrained  by  the  resistance 
of  the  central  and  outer  layers  of  material  which  are 
still  cold ;  the  result  of  this  contest  is,  that  while  the  inner 


24  GLASS  MANUFACTURE. 

layer  is  thrown  into  a  state  of  compression,  the  outer  and 
central  layers  are  thrown  into  a  state  of  tension.  Accord- 
ingly, if  the  tension  so  produced  is  sufficiently  great,  the 
outer  layers  fracture  under  tension  and  the  whole  vessel 
is  shattered  by  the  propagation  of  the  crack  thus  initiated. 
From  this  description  of  the  process  it  will  be  seen  that  a 
high  coefficient  of  expansion  and  alow  modulus  of  elasticity 
will  both  favour  fracture,  while  high  tensile  strength  will 
tend  to  prevent  it.  The  thermal  conductivity  of  the  glass 
will  also  affect  the  result,  because  the  intensity  of  the 
tensile  stress  set  up  in  the  colder  layers  of  glass  will 
depend  upon  the  temperature  gradient  which  exists  in  the 
glass ;  thus  if  glass  were  a  good  conductor  of  heat  it  would 
never  be  possible  to  set  up  a  sufficient  difference  of 
temperature  between  adjacent  layers  to  produce  fracture ; 
for  the  same  reason,  vessels  of  very  thin  glass  are  less  apt 
to  break  under  temperature  changes  than  those  having 
thick  walls,  since  the  greatest  difference  of  temperature  that 
can  be  set  up  between  the  inner  and  outer  layers  of  a  thin- 
walled  vessel  can  never  be  very  considerable.  It  also 
follows  from  these  considerations,  that  if  a  cold  glass  vessel 
be  simultaneously  heated  or  cooled  from  both  sides,  it  can  be 
safely  exposed  to  a  much  more  sudden  change  of  temperature 
than  it  could  withstand  if  heated  from  one  side  alone ;  on 
the  other  hand,  when  very  thick  masses  of  glass  have  to 
be  heated,  this  must  be  done  very  gradually,  as  a  con- 
siderable time  will  necessarily  elapse  before  an  increment 
of  temperature  applied  to  the  outside  will  penetrate  to  the 
centre  of  the  mass.  It  should  also  be  noted  here,  that  in 
addition  to  the  thermal  conductivity  of  the  glass,  its  heat 
capacity  or  specific  heat  also  enters  into  this  question,  since 


THE  PHYSICAL  PROPERTIES  OF   GLASS.  25 

heat  will  obviously  penetrate  more  slowly  through  a  glass 
whose  own  rise  of  temperature  absorbs  a  greater  quantity 
of  heat.  It  will  thus  be  seen  that  "  thermal  endurance  " 
is  a  somewhat  complicated  property,  depending  upon  the 
factors  named  above,  viz. :  coefficient  of  expansion,  thermal 
conductivity,  specific  heat,  Young's  modulus  of  elasticity, 
and  tensile  strength. 

The  coefficient  of  thermal  expansion  varies  considerably 
in  different  glasses,  and  we  can  here  only  state  the  limiting 
values  between  which  these  coefficients  usually  lie ;  these 
are  37  x  10  ~7  as  the  loweri  and  122  x  10  ~7  as  the  upper 
limit.  These  figures  express  the  cubical  expansion  of  the 
glass  per  degree  Centigrade,  the  corresponding  figures  for 
steel  and  brass  respectively  being  about  360  x  10  ~7  and 
648  x  10  ~7  respectively.  It  should  be  noted  that  vitreous 
bodies  of  extremely  low  expansibility  are  obtainable  by  the 
suitable  choice  of  ingredients,  but  in  some  cases  these 
"  glasses  "  are  white  opaque  bodies,  and  in  all  cases  they 
present  great  difficulty  in  manufacture,  owing  to  the  fact 
that  alkalies  and  lime  must  be  avoided  in  their  composition. 

Quite  apart  from  the  question  of  thermal  endurance,  the 
expansive  properties  of  glass  are  of  some  importance. 
Thus  when  several  kinds  of  glass  have  to  be  united,  as, 
for  example,  in  the  process  of  producing  "flashed"  coloured 
glass,  it  is  essential  that  their  coefficients  of  expansion 
should  be  as  nearly  as  possible  the  same ;  otherwise  con- 
siderable stresses  will  be  set  up  when  the  glasses,  which 
have  been  joined  at  a  red  heat,  are  allowed  to  cool.  On 
the  other  hand,  this  mutual  stressing  of  two  glasses  owing 
to  differences  in  their  thermal  expansion  has  been  utilised 
for  the  production  of  tubes  and  other  glass  objects  possess- 


26  GLASS  MANUFACTUEE. 

ing  special  strength.  If  a  tube  be  drawn  out  of  glass 
consisting  of  two  layers,  one  considerably  more  expansible 
than  the  other,  and  the  cooling  process  be  rightly  conducted, 
it  is  possible  to  produce  a  tube  in  which  both  the  inner  and 
outer  layers  of  glass  are  under  a  considerable  compressive 
stress.  Not  only  is  glass,  as  we  have  seen  above,  enormously 
stronger  as  against  compression  than  it  is  against  tension, 
but  glass  under  compressive  stress  behaves  as  though  it 
were  a  much  tougher  material,  being  less  liable  to  injury 
by  scratches  or  blows.  Moreover,  if  a  tube  in  this  condition 
be  heated  and  then  exposed  to  sudden  cooling,  the  first 
effect  of  the  application  of  cold  will  be  a  contraction  of  the 
surface  layers,  resulting  in  a  relief  of  the  initial  condition 
of  compression.  These  tubes  are,  therefore,  remarkably 
indifferent  to  sudden  cooling,  although  they  are  naturally 
more  sensitive  to  sudden  heating.  In  this  respect  they 
differ  entirely  from  ordinary  glass,  which  is  considerably 
more  sensitive  to  sudden  cooling  than  to  sudden  heating, 
particularly  when  the  heat  or  cold  is  applied  to  all  the 
surfaces  of  the  object  at  the  same  time.  The  special  tubes 
made  of  two  layers  of  glass  above  referred  to  are  manu- 
factured by  the  Jena  Glass  Works  for  special  purposes, 
among  which  boiler  gauge  glasses  are  the  most  important. 
It  should  be  also  mentioned  here  that  the  remarkable 
thermal  endurance  of  vitrified  silica,  which  can  be  raised 
to  a  red  heat  and  then  immersed  in  cold  water  without  risk 
of  breakage,  is  chiefly  due  to  its  very  low  coefficient  of 
expansion. 

In  another  direction  the  expansive  properties  of  glass  are 
of  importance  wherever  glass  is  rigidly  attached  to  metal. 
At  the  present  time  this  is  done  in  several  industrial 


THE  PHYSICAL  PEOPEETIES  OF  GLASS.  27 

products,  such  as  incandescent  electric  lamps  and  "  wired  " 
plate  glass.  In  certain  varieties  of  incandescent  lamps, 
metallic  wires  are  sealed  into  the  glass  bulbs,  and  the  only 
metal  available  for  this  purpose,  at  all  events  until  recently, 
has  been  platinum,  whose  coefficient  of  expansion  is  low 
as  compared  with  most  metals,  and  whose  freedom  from 
oxidation  when  heated  to  the  necessary  temperature  makes 
it  easy  to  produce  a  clean  joint  between  glass  and  metal. 
More  recently  the  use  of  certain  varieties  of  nickel  steel  has 
been  patented  for  this  purpose,  since  it  is  possible  to  obtain 
nickel  steel  alloys  of  almost  any  desired  coefficient  of 
expansion  from  that  of  the  alloy  known  as  "  invar,"  having 
a  negligibly  small  expansion  compared  with  that  of  ordinary 
steel.  By  choosing  a  suitable  member  of  this  series  a  metal 
could  be  obtained  whose  coefficient  of  expansion  corresponds 
exactly  with  that  of  the  glass  to  which  it  is  to  be  united. 
The  oxidation  of  the  nickel  steel  when  heated  to  the 
temperature  necessary  for  effecting  its  union  with  the  glass 
presented  serious  difficulties  to  the  production  of  a  tight 
joint,  and  several  devices  for  avoiding  this  oxidation  have 
been  patented.  In  the  incandescent  electric  lamp,  although 
the  joint  between  glass  and  metal  is  required  to  be  perfectly 
air-tight,  the  two  bodies  are  only  attached  to  one  another 
over  a  very  short  length.  In  wired  plate  glass,  however, 
an  entire  layer  of  wire  netting  is  interposed  between  two 
layers  of  glass,  the  wire  being  inserted  during  the  process 
of  rolling.  Here  a  certain  amount  of  oxidation  of  the  wire 
is  not  of  any  serious  importance,  as  it  only  appears  to  give 
rise  to  a  few  bubbles,  whose  presence  does  not  interfere 
with  the  strength  and  usefulness  of  the  glass  ;  but  any 
considerable  difference  of  coefficient  of  expansion  will 


28  GLASS  MANUFACTUEE. 

produce  the  most  serious  results  on  account  of  the  great 
lengths  of  glass  and  metal  that  are  attached  to  each  other. 
This  factor  has  been  neglected  hy  some  manufacturers,  with 
the  result  that  much  of  the  wired  glass  of  commerce  is 
liable  to  crack  spontaneously  some  time  after  it  has  left  the 
manufacturer's  hands,  while  there  is  also  much  loss  by 
breakage  during  the  process  of  manufacture. 

Thermal  expansion  is  a  vital  factor  in  yet  another  of  the 
uses  of  glass.  Our  ordinary  instrument  for  measuring 
temperature — the  mercury  thermometer — is  very  consider- 
ably affected  by  the  expansive  behaviour  of  glass.  When 
a  mercury  thermometer  is  warmed  the  mercury  column 
rises  in  the  stem  because  the  mercury  expands  upon 
warming  to  a  greater  extent  than  the  glass  vessel,  bulb 
and  stem,  in  which  it  is  contained.  The  subject  of  the 
graduations  and  corrections  of  the  mercury  glass  ther- 
mometer is  a  very  large  one  and  somewhat  outside  the 
scope  of  the  present  volume ;  but  attention  should  be 
drawn  in  this  place  to  the  peculiarities  of  the  behaviour  of 
glass  that  have  been  discovered  in  this  connection.  One 
of  these  is  that  when  first  blown  the  bulb  of  a  thermometer 
takes  a  very  considerable  time  to  acquire  its  final  volume, 
the  result  being,  that  if  a  freshly  made  thermometer  is 
graduated,  after  some  time  the  zero  of  the  instrument  will 
be  found  considerably  changed,  generally  in  a  direction 
which  indicates  that  the  volume  of  the  bulb  has  slightly 
increased.  By  a  special  annealing  or  "  ageing  "  process 
this  change  can  be  completed  in  a  comparatively  short  time 
before  the  instrument  is  graduated.  There  is,  however,  a 
further  peculiarity  \vhich  is  prominent  in  some  thermometers, 
although  very  greatly  reduced  in  the  best  modern  glasses. 


THE  PHYSICAL  PKOPERTIES   OF  GLASS.  29 

This  becomes  apparent  in  a  decided  change  of  zero  when- 
ever the  thermometer  has  been  exposed  for  any  length  of 
time  to  a  high  temperature,  the  zero  gradually  returning 
more  or  less  to  its  original  position  in  the  course  of  time. 
With  thermometers  made  of  glasses  liable  to  these  aberra- 
tions, the  reading  for  a  given  temperature  depended  largely 
upon  the  immediate  past  history  of  the  instrument ;  but, 
thanks  to  the  Jena  Works,  thermometer  glasses  are  now 
available  which  are  almost  entirely  free  from  this  defect. 
In  this  connection  the  curious  fact  has  been  observed  that 
glass  containing  both  the  alkalies  (potash  and  soda)  shows 
these  thermal  effects  much  more  markedly  than  a  glass 
containing  one  of  the  alkalies  only. 

The  thermal  conductivity  of  glass,  except  in  so  far  as  it 
affects  the  thermal  endurance,  is  not  a  matter  of  any  great 
direct  practical  importance,  although  the  fact  that  glass  is 
alwajs  a  comparatively  poor  conductor  of  heat  is  utilised 
in  many  of  its  applications,  as,  for  example,  the  construction 
of  conservatories  and  hot-houses,  although  even  in  that 
case  the  opacity  of  glass  to  thermal  radiations  of  long  wave 
lengths  is  of  more  importance  than  its  low  thermal  con- 
ductivity. Similar  statements  apply,  in  a  still  more  marked 
degree,  to  the  subject  of  the  specific  heat  of  glass. 

The  electrical  properties  of  glass  are  of  much  greater 
practical  importance,  glass  being  frequently  used  in 
electrical  appliances  as  an  insulating  medium.  The 
insulating  properties  of  glass,  as  well  as  the  property 
known  as  the  specific  inductive  capacity,  vary  greatly 
according  to  the  chemical  composition  of  the  material. 
Generally  speaking,  the  harder  glasses,  i.e.,  those  richest 
in  silica  and  lime,  are  the  best  insulators,  while  soft  glasses, 


30  GLASS  MANUFACTURE. 

rich  in  lead  or  alkali,  are  much  poorer  in  this  respect.  In 
practice,  particularly  when  the  glass  insulator  is  exposed 
to  even  a  moderately  damp  atmosphere,  the  nature  of  the 
glass  affects  the  resulting  insulation  or  absence  of  insula- 
tion, in  another  way.  Almost  all  varieties  of  glass  have  the 
property  of  condensing  upon  their  surfaces  a  decided  film 
or  layer  of  moisture  from  the  atmosphere,  and,  as  we  have 
seen  above,  glasses  differ  very  considerably  in  the  degree 
to  which  they  display  this  hygroscopic  tendency.  The 
softer  glasses  are  much  more  hygroscopic  than  the  hard 
ones,  and  the  resulting  film  of  surface  moisture  serves  to 
lessen  or  even  to  break  down  the  insulating  power  of  the 
glass,  the  electricity  leaking  away  along  the  film  of  moisture. 
In  the  case  of  appliances  for  static  electricity,  where  very 
high  voltages  have  to  be  dealt  with,  an  endeavour  is  some- 
times made  to  avoid  this  leakage  by  varnishing  the  surface 
of  the  glass  with  shellac  or  other  similar  substance,  and 
this  proves  a  satisfactory  remedy  up  to  a  certain  point. 
Quite  recently  a  variety  of  glass  has  been  brought  forward 
which  is  peculiar  in  having  a  comparatively  low  electrical 
resistance,  so  that  for  certain  purposes  it  can  be  used  as 
an  electric  conductor.  Although  interesting  in  itself,  this 
glass  is  not  very  likely  to  prove  useful  even  for  the  limited 
number  of  applications  that  could  be  found  for  an  electri- 
cally conducting  glass,  since  it  is  very  rich  in  alkali,  and 
is,  therefore,  likely  to  be  unstable  chemically,  even  under 
the  action  of  atmospheric  agencies  alone. 

The  most  valuable  and  in  many  ways  the  most  in- 
teresting of  the  properties  of  glass — its  transparency — has 
not  been  dealt  with  as  yet,  and  all  mention  of  this  subject 
has  been  postponed  to  the  end  of  the  present  chapter, 


THE  PHYSICAL  PEOPEETIES  OF  GLASS.  31 

because  the  whole  subject  of  the  optical  properties  of  glass 
will  be  dealt  with  more  fully  in  the  chapter  on  optical  glass 
(Chap.  XII.),  so  that  a  very  brief  reference  only  need  be 
made  to  the  matter  here. 

There  can  be  no  doubt  that,  in  most  of  its  practical  appli- 
cations, transparency  is  the  fundamental  and  essential 
property  which  leads  to  the  employment  of  glass  in  the 
place  of  either  stronger  or  cheaper  materials.  By  trans- 
parency, in  this  sense,  we  wish  to  include  mere  translucence 
also,  since  very  frequently  it  is  as  necessary  to  avoid  un- 
disturbed visibility  as  it  is  to  secure  the  admission  of  light. 
It  is  indeed  hard  to  find  any  use  to  which  glass  is  extensively 
put  into  which  the  function  of  transmitting  light  does  not 
very  largely  enter.  Almost  the  only  such  example  of  use  is 
the  modern  application  of  opal  glass  to  the  covering  of  walls, 
and  the  use — not  as  yet  widely  extended — of  pressed  glass 
blocks  as  bricks  and  paving  stones  ;  in  these  cases  it  is 
the  hardness  and  smoothness  of  surface  that  gives  to  the 
vitreous  body  its  superiority  over  other  materials,  but  apart 
from  these  special  cases,  the  fact  remains  that  well  over 
95  per  cent,  of  the  glass  used  in  the  world  is  employed  for 
purposes  where  transmission  of  light  is  essential  to  the 
attainment  of  the  desired  result,  either  from  the  point  of 
view  of  utility  or  from  that  of  beauty.  It  is  interesting  to 
note  that  the  power  of  transmitting  light  is  not  shared  by 
many  solid  bodies.  Some  colloidal  organic  bodies,  such  as 
gelatine  and  celluloid,  possess  the  property  to  a  degree  com- 
parable with  glass,  while  certain  mineral  crystals,  such  as 
quartz  and  fluor-spar,  may  even  surpass  the  finest  glass  in 
this  respect;  while  some  of  the  other  optical  properties  of 
glass  are  greatly  exceeded  by  such  natural  substances  as 


32  GLASS  MANUFACTUEE. 

the  diamond  and  the  ruby.  But  the  very  brevity  of  this 
list  is  in  itself  striking,  because  it  must  be  borne  in  mind 
that  transparency  by  no  means  constitutes  the  only 
common  characteristic  of  vitreous  bodies. 

Although  the  transparency  of  glass  is  so  valuable  and 
indeed  so  essential  a  property  of  that  substance,  it  must  be 
remembered  that  no  kind  of  glass  is  perfectly  transparent. 
Quite  apart  from  the  fact  that  of  the  light  that  falls  upon  a 
glass  surface,  however  perfectly  polished,  a  considerable 
proportion  is  turned  back  by  reflection  at  the  surface  of 
entry  and  again  by  reflection  at  the  surface  of  exit  from 
the  glass,  a  certain  proportion  of  light  is  absorbed  during 
its  passage  through  the  glass  itself,  and  the  transmitted 
beam  is  correspondingly  weakened.  In  the  purest  and 
best  glasses  this  absorption  is  so  small  that  in  any  moderate 
thickness  very  delicate  instruments  are  required  to  show 
that  there  has  been  any  loss  of  light  at  all ;  but  even  the 
best  glass,  when  examined  through  a  thickness  of  20  in. 
or  more,  always  shows  the  effects  of  the  absorption  of  light 
quite  unmistakably.  In  fact,  not  only  does  all  glass 
absorb  light,  but  it  does  this  to  a  different  degree  accord- 
ing to  the  colour  of  the  light,  so  that  in  passing  through 
the  glass  a  beam  of  white  light  becomes  weakened  in  one 
of  its  constituent  colours  more  than  in  the  others,  with  the 
result  that  the  emergent  light  is  slightly  coloured.  Thus 
the  purest  and  whitest  of  glasses,  when  examined  in  very 
thick  pieces,  always  show  a  decided  blue  or  green  tint, 
although  this  tint  is  quite  invisible  on  looking  through  a 
few  inches  of  the  glass.  The  ordinary  glass  of  commerce, 
however,  is  far  removed  from  even  this  approach  to  perfect 
transparency.  The  best  plate  glass  shows  a  slight  greenish- 


THE  PHYSICAL  PEOPERTIES   OF   GLASS.  33 

blue  tint,  which  is  just  perceptible  to  the  trained  eye 
when  a  single  sheet  of  moderate  thickness  is  laid  down 
upon  a  piece  of  white  paper.  When  a  sheet  of  this  glass 
is  viewed  edgewise,  in  such  a  way  that  the  light  reaching 
the  eye  has  traversed  a  considerable  thickness,  the  greenish- 
blue  tint  of  the  glass  becomes  more  apparent.  By 
holding  strips  of  various  kinds  of  glass,  cut  to  an  equal 
length,  close  together  and  comparing  the  colour  exhibited 
by  their  ends,  a  means  of  comparing  the  colours  of 
apparently  "  white  "  glasses  is  readily  obtained.  It  will  be 
found  that  different  specimens  of  glass  differ  most  markedly 
in  this  respect.  Sheet  glass  is,  as  a  rule,  decidedly  deeper 
in  colour  than  polished  plate,  but  rolled  plate  is  as  a  rule 
much  greener — the.  colour  of  this  glass  can,  in  fact,  in  most 
cases  be  seen  quite  plainly  in  looking  through  or  at  the 
sheets  in  the  ordinary  way. 

The  question  of  how  far  the  colour  of  glass  affects  the 
value  of  the  light  which  it  transmits  depends  for  its  answer 
upon  the  purpose  to  which  the  lighted  space  is  to  be  put. 
Where  delicate  comparisons  of  colour  are  to  be  made,  or 
other  delicate  work  involving  the  use  of  the  colour  sense  is 
to  be  carried  on,  it  is  essential  that  all  colouration  of  the 
entering  daylight  should  be  avoided,  and  the  use  of  the 
most  colourless  glass  obtainable  will  be  desirable.  Again, 
in  photographic  studios  it  is  important  to  secure  a  glass 
which  shall  absorb  as  small  a  proportion  of  the  chemically 
active  rays  contained  in  daylight  as  possible,  and  special 
glasses  for  this  purpose  are  available.  Although  for  the 
present  the  price  of  these  special  glasses  may  prove  pro- 
hibitive for  the  glazing  of  studio  lights,  their  use  is  found 
highly  advantageous  where  artificial  light  is  to  be  used  to 

G.M.  D 


34  GLASS  MANUFACTUEE. 

the  best  advantage.  On  the  other  hand,  for  every-day 
purposes,  the  slight  tinge  of  colour  introduced  into  the 
light  by  the  colour  of  ordinary  sheet  and  plate  glass,  or 
even  of  greenish  rolled  plate  glass,  has  no  deleterious  effect 
whatever,  the  majority  of  persons  being  entirely  unconscious 
of  its  presence.  The  transmission  of  light  by  glass,  its 
absorption,  refraction,  dispersion,  etc.,  are,  however,  best 
grouped  together  as  the  "  optical "  properties  of  glass,  and 
under  that  heading  they  will  receive  a  fuller  treatment  in 
connection  with  the  subject  of  the  manufacture  of  glass  for 
optical  purposes. 


CHAPTEE   III. 

THE    RAW    MATERIALS    OF    GLASS    MANUFACTURE. 

THE  choice  of  raw  materials  for  all  branches  of  glass 
manufacture  is  a  matter  of  vital  importance.  As  a  rule  all 
''fixed"  bodies  that  are  once  introduced  into  the  glass- 
melting  pot  or  furnace  appear  in  the  finished  glass,  while 
volatile  or  combustible  bodies  are  more  or  less  completely 
eliminated  during  the  process  of  fusion.  Thus  while  the 
chemical  manufacturer  can  purify  his  products  by  filtration, 
crystallisation  or  some  other  process  of  separation,  the 
glass-maker  must  eliminate  all  undesirable  ingredients 
before  they  are  permitted  to  enter  the  furnace,  and  the 
stringency  of  this  condition  is  increased  by  the  fact  that 
the  transparency  of  glass  makes  the  detection  of  defects  of 
colour  or  quality  exceedingly  easy.  For  the  production  of 
the  best  varieties  of  glass,  therefore,  an  exacting  standard 
of  purity  is  applied  to  the  substances  used  as  raw  materials. 
As  the  quality  of  the  product  decreases,  so  also  do  the 
demands  upon  the  purity  of  raw  materials,  until  finally 
for  the  manufacture  of  common  green  bottles,  even  such 
very  heterogeneous  substances  as  basaltic  rock  and  the 
miscellaneous  residues  of  broken,  defective  and  half-melted 
glass  forming  the  refuse  of  other  glassworks  may  be 
utilised  more  or  less  satisfactorily. 

D  2 


36  GLASS  MANUFACTURE. 

For  the  best  kinds  of  glass  the  most  desirable  quality  in 
raw  materials  is  thus  as  near  an  approach  to  purity  as 
possible  under  commercial  conditions,  and  next  to  that,  as 
great  a  constancy  of  composition  as  possible.  For  instance, 
the  quantity  of  moisture  contained  in  a  ton  of  sand 
appreciably  affects  the  resulting  composition  of  the  glass, 
and  if  the  sand  cannot  be  obtained  perfectly  dry,  it  should  at 
least  contain  a  constant  proportion  of  moisture,  otherwise  it 
becomes  necessary  to  determine,  by  chemical  tests,  the  per- 
centage of  moisture  in  the  sand  that  is  used  from  day  to  day, 
and  to  adjust  the  quantityused  in  accordance  with  the  results 
of  these  tests,  a  proceeding  which,  of  course,  materially  com- 
plicates the  whole  process.  In  other  cases,  variable  com- 
position is  not  so  readily  allowed  for,  and  uncontrollable 
variations  in  the  composition  of  the  glass  result — at  times 
the  quality  falls  off  unaccountably,  or  the  glass  refuses  to 
melt  freely  at  the  usual  temperature.  The  systematic 
employment  of  chemical  analysis  in  the  supervision  of  both 
the  raw  materials  and  of  various  products  will  frequently 
enable  the  manufacturer  to  trace  the  causes  of  such  un- 
desirable occurrences;  but  however  necessary  such  control 
undoubtedly  is,  it  cannot  entirely  compensate  for  the  use  of 
raw  materials  liable  to  too  great  a  variation  in  composition 
or  physical  character.  For  not  only  the  chemical  com- 
position, but  also  the  physical  condition  and  properties  of 
the  material  are  of  importance  in  glass  manufacture.  Thus 
it  is  essential  that  materials  to  be  used  for  glass-melting 
should  be  obtainable  in  a  reasonably  fine  state  of  division, 
and  in  this  connection  it  must  be  remembered  that  both 
exceedingly  hard  bodies  and  soft  plastic  substances  can 
only  be  ground  with  very  great  difficulty.  Further,  where 


THE  RAW  MATERIALS  OF   GLASS  MANUFACTURE.     37 

a  substance  occurs  naturally  as  a  powder,  this  powder 
should  be  of  uniform  and  not  too  fine  a  grain,  more 
especially  if  it  belongs  to  the  class  of  refractory  rather 
than  of  fluxing  ingredients.  In  that  case  the  presence  of 
coarser  grains  will  result  in  their  presence  in  the  undis- 
solved  state  in  the  finished  glass,  unless  excessive  heat  and 
duration  of  "  founding  "  be  employed  to  permit  of  their 
dissolution.  This  applies  chiefly  to  siliceous  and  calcareous 
ingredients,  but  hardened  nodules  of  salt-cake  may  behave 
in  a  similar  manner. 

A  further  consideration  in  the  choice  of  raw  materials  is 
facility  of  storage.  Thus  limestone  in  the  shape  of  large 
lumps  of  stone  which  are  only  ground  to  powder  as 
required,  is  readily  stored,  and  undergoes  no  deleterious 
change  even  if  exposed  to  the  weather  ;  on  the  other  hand, 
sulphate  of  soda  (salt-cake),  if  stored  even  in  moderately 
dry  places,  rapidly  agglomerates  into  hard  masses,  at  the 
same  time  absorbing  a  certain  percentage  of  moisture. 
Such  properties  are  not  always  to  be  avoided,  salt-cake  for 
example  being  at  the  present  time  an  indispensable  in- 
gredient in  many  kinds  of  glass-making,  but  the  value  of  a 
substance  is  in  some  cases  materially  lessened  by  such 
causes. 

The  raw  materials  ordinarily  employed  in  glass-making 
may  be  grouped  into  the  following  classes  : — 

(1)  Sources  of  silica. 

(2)  Sources  of  alkalies. 

(3)  Sources  of  bases  other  than  alkalies. 

(1)  Sources  of  Silica. — The  principal  source  of  silica  is 
sand.  This  substance  occurs  in  nature  in  geological 


38  GLASS   MANUFACTURE. 

deposits,  often  of  very  considerable  area  and  depth. 
These  deposits  of  sand  have  always  been  formed  by  the 
disintegration  of  a  siliceous  rock,  and  the  fragments  so 
formed  have  been  sifted  and  transported  by  the  agency  of 
water,  being  finally  deposited  by  a  river  either  in  the  sea 
(marine  deposits)  or  in  lakes  (lacustrine  deposits),  while  the 
action  of  the  water,  either  during  transport  or  after  deposi- 
tion, has  frequently  worn  the  individual  particles  into  the 
shape  of  rounded  grains. 

In  consequence  of  this  origin,  the  chemical  composition 
of  sand  varies  very  greatly  with  the  nature  of  the  rock 
whose  denudation  gave  rise  to  the  deposit.  Where  rocks 
very  rich  in  silica,  or  even  consisting  of  nearly  pure  silica, 
have  been  thus  denuded,  the  resulting  sand  is  often  very 
pure,  deposits  containing  up  to  99'9  per  cent,  silica  being 
known.  More  frequently,  however,  the  sand  contains 
fragments  of  more  or  less  decomposed  felspar,  which 
introduce  alumina,  iron  and  alkalies  into  its  composition. 
Finally,  "  sands"  of  all  ranges  of  composition  from  the 
pure  varieties  just  referred  to  down  to  the  clay  marls,  very 
rich  in  iron  and  alumina,  are  known. 

For  the  best  varieties  of  glass,  viz.,  optical  glass,  flint 
glass  and  the  whitest  sheet-glass,  as  well  as  for  the  best 
Bohemian  glass,  a  very  pure  variety  of  sand  is  required, 
preferably  containing  less  than  0*05  per  cent,  of  iron, 
and  not  more  than  0'05  per  cent,  of  other  impurities  such 
as  alumina,  lime  or  alkali.  As  a  matter  of  fact,  sands 
containing  so  little  iron  rarely  contain  any  other  impurity 
except  alumina  in  measurable  quantities.  The  best-known 
deposit  of  such  sand  in  Europe  is  that  at  Fontainebleau 
near  Paris,  but  equally  good  sand  is  found  at  Lippe  in 


THE  KAW  MATEEIALS   OF   GLASS  MANUFACTURE.     39 

Germany,  whence  sand  is  delivered  commercially  with 
a  guaranteed  silica  content  of  99'98  per  cent.  Sand  of 
excellent  quality,  although  not  quite  so  good  as  the  above, 
is  obtained  at  Hohenbocka  in  Germany  (Saxony)  and  at  a 
few  other  places  in  Europe.  In  England  no  deposit  of  sand 
of  such  purity  is  at  present  being  exploited. 

Next  in  order  of  value  to  these  exceedingly  pure  sands, 
come  the  glass-making  sands  of  Belgium,  notably  of  Epinal. 
These  usually  contain  from  0'2  to  0*3  per  cent,  of  iron  and 
rather  more  alumina,  but  they  are  used  very  largely  for  the 
manufacture  of  sheet  and  plate-glass.  When  the  standard 
of  quality  is  further  relaxed,  a  large  number  of  sand 
deposits  become  available,  and  the  manufacturers  of  each 
district  avail  themselves  of  more  or  less  local  supplies ;  thus 
in  England  the  sands  of  Leighton  in  Bedfordshire  and  of 
Lynn  on  the  East  Coast,  are  largely  used.  Finally,  for  the 
manufacture  of  the  cheapest  class  of  bottles,  sands  contain- 
ing up  to  2  per  cent,  of  iron  and  a  considerable  proportion 
of  other  substances  are  employed. 

Silica,  in  various  states  of  purity,  occurs  in  nature  in  a 
number  of  other  forms  than  that  of  sand.  By  far  the 
commonest  of  these  is  that  of  more  or  less  compact  sedi- 
mentary rock,  known  as  "  sandstone."  As  far  as  chemical 
composition  is  concerned,  some  of  these  stones  are  admirably 
suited  for  making  the  best  kinds  of  glass,  although  as  a  rule 
a  stone  is  not  so  homogeneous  as  the  material  of  a  good 
sand-bed.  The  stone  has  the  further  disadvantage  that  it 
requires  to  be  crushed  to  powder  before  it  can  be  used  for 
glass-making,  and  the  crushed  product  is  generally  a 
mixture  of  grains  of  all  sizes  ranging  from  a  fine  dust  to  the 
largest  size  of  grain  passed  by  the  sieves  attached  to  the 


40  GLASS  MANUFACTURE. 

crushing  machine.  The  presence  of  the  very  fine  particles 
is  a  distinct  objection  from  the  glass-maker's  point  of  view, 
so  that  it  would  probably  be  necessary  to  wash  the  sand  so 
as  to  remove  this  dust — a  process  that  in  itself  adds  to  the 
cost  of  the  crushed  stone  and  at  the  same  time  leads  to  the 
loss  of  a  serious  percentage  of  the  material.  Objections  of 
the  same  kind  apply,  but  with  still  greater  force,  to  the  use 
of  powdered  quartz  or  flint  as  sources  of  silica  for  the 
glass-maker  ;  further,  these  materials  are  exceedingly  hard 
and  therefore  difficult  to  crush,  so  that  the  price  of  the 
materials  is  prohibitive  for  glass-making  purposes.  The 
use  of  ground  quartz  and  flint  is  therefore  confined  to  the 
ceramic  industries  in  which  these  substances  serve  as  sources 
of  silica  for  both  bodies  and  glazes ;  in  former  times,  how- 
ever, ground  flint  was  extensively  used  in  the  manufacture 
of  the  best  kinds  of  glass,  as  the  still  surviving  name  of 
"  flint  glass  "  testifies. 

Minerals  of  the  felspar  class,  consisting  essentially  of 
silicates  of  alumina  and  one  or  more  of  the  alkalies,  are 
extensively  used  in  glass-making  and  should  be  mentioned 
here,  since  their  high  silica-content  (up  to  70  per  cent.) 
constitutes  an  effective  source  of  silica.  As  a  source  of  this 
substance,  however,  most  felspars  would  be  far  too  expensive, 
and  their  use  is  due  to  their  content  of  alumina  and  alkali. 

(2)  Sources  of  Alkali. — Originally  the  alkaline  constituents 
of  glass  were  derived  from  the  ashes  of  plants  and  of  seaweed 
or  "  kelp  " ;  in  both  cases  the  alkali  was  obtained  in  the 
form  of  carbonate  and  was  ordinarily  used  in  a  very  impure 
form  :  at  the  present  time,  however,  the  original  source  of 
alkali  for  industrial  purposes  is  found  in  the  natural  deposits 
and  other  sources  of  the  chlorides  of  sodium  and  potassium. 


THE  RAW  MATEEIALS   OF   GLASS  MANUFACTUKE.     41 

At  the  present  time  it  is  not  yet  industrially  possible  to 
introduce  the  alkalies  into  glass  mixtures  in  the  natural 
form  of  chlorides.  The  principal  difficulty  in  doing  this  arises 
from  the  fact  that  the  chlorides  are  volatile  at  the  tempera- 
ture of  glass-melting  furnaces  and  are  only  acted  upon  by 
hot  silica  in  the  presence  of  water  vapour.  Introduced  into 
an  ordinary  glass  furnace,  therefore,  these  salts  would  be 
driven  off  as  vapour  before  they  could  combine  with  the 
other  ingredients  in  the  desired  form  of  double  silicates. 

Alkalies  are,  therefore,  introduced  into  the  glass  mixture 
in  less  volatile  and  more  readily  attackable  forms.  Of 
these  the  carbonate  is  historically  the  earlier,  while  the 
sulphate  is  at  the  present  time  industrially  by  far  the  more 
important.  The  Carbonate  of  Soda,  or  soda  ash,  which  is 
used  in  the  production  of  some  special  glasses,  and  is  an 
ingredient  of  English  flint  glasses,  is  produced  by  either  of 
two  well-known  chemical  processes.  One  of  these  is  the 
"  black  ash,"  or  "  Le  Blanc  "  process,  in  which  the  chloride 
is  first  converted  into  sulphate  by  the  direct  action  of  sul- 
phuric acid,  and  the  sulphate  thus  formed  is  converted  into 
the  carbonate  by  calcination  with  a  mixture  of  calcium 
carbonate  and  coal.  The  sodium  carbonate  thus  formed  is 
separated  by  solution  and  subsequent  evaporation.  A  purer 
form  of  sodium  carbonate  can  be  obtained  with  great 
regularity  by  the  "  ammonia  soda  "  process,  in  which  a 
solution  of  sodium  chloride  is  acted  upon  by  ammonia  and 
carbonic  acid  under  pressure.  Soda  ash  produced  by  this 
process  is  now  supplied  regularly  for  glass-making  purposes 
in  a  state  of  great  purity  and  constancy  of  composition. 
It  is  upon  these  qualities  that  the  great  advantages  of  this 
substance  depend,  since  its  relatively  high  cost  precludes 


42  GLASS  MANUFACTUEE. 

its   use  except  for  special  kinds  of   glass,    and  for   these 
purposes  the  qualities  named  are  of  great  value. 

For  most  purposes  of  glass-making,  such  as  the  produc- 
tion of  sheet  and  plate-glass  of  all  kinds,  the  alkali  is 
introduced  in  the  form  of  salt-cake— i.e.,  sulphate  of  soda. 
This  product  is  obtained  as  the  result  of  the  first  step  of 
the  Le  Blanc  process  of  alkali  manufacture — i.e.,  hy  the 
action  of  sulphuric  acid  on  sodium  chloride  ;  salt-cake  is 
thus  a  relatively  crude  product,  and  its  use  is  due  to  the 
fact  that  it  is  by  far  the  cheapest  source  of  alkali  available 
for  glass-making.  There  are,  however,  certain  disadvantages 
connected  with  its  use,  The  chief  of  these  is  the  fact  that  silica 
cannot  decompose  salt-cake  without  the  aid  of  a  reducing 
agent ;  such  a  reducing  agent  is  partly  supplied  by  the 
flame-gases  in  the  atmosphere  of  the  furnace,  but  in 
addition  to  these  a  certain  proportion  of  carbon,  in  the 
form  of  coke,  charcoal  or  anthracite  coal  must  be  added  to 
all  glass  mixtures  containing  salt-cake.  The  use  of  a 
slightly  incorrect  quantity  of  carbon  for  this  purpose  leads 
to  disastrous  results,  while  even  under  the  best  conditions 
it  is  not  easy  to  remove  all  traces  of  sulphur  compounds 
from  glass  made  in  this  way.  A  further  risk  of  trouble 
arises  in  connection  with  salt-cake  from  the  fact  that  it  is 
never  entirely  free  from  more  or  less  deleterious  impurities, 
According  to  the  exact  manner  in  which  it  has  been  pre- 
pared, the  substance  always  contains  a  small  excess  either 
of  undecomposed  sodium  chloride  or  of  free  sulphuric  acid, 
or  the  latter  may  be  present  in  the  form  of  sulphate  of 
lime.  A  good  salt-cake,  however,  should  contain  at  least 
97  per  cent,  of  anhydrous  sodium  sulphate,  and  not  more 
than  1*0  per  cent,  of  either  sodium  chloride  or  sulphuric 


THE   KAW  MATEEIALS   OF   GLASS  MANUFACTUKE.     43 

acid.  While  pure  sodium  sulphate  is  readily  soluble  in 
water,  ordinary  salt-cake  always  leaves  an  insoluble  residue, 
consisting  frequently  of  minute  particles  of  clay  or  other 
material  derived  from  the  lining  of  the  furnace  in  which  it 
was  prepared,  or  from  the  tools  with  which  it  was  handled ; 
and  these  impurities  are  liable  to  become  deleterious  to  the 
glass  if  present  in  any  quantity.  The  insoluble  residue 
should  not  exceed  0'5  per  cent,  in  amount,  and  in  the  best 
salt-cake  is  generally  under  0*2  per  cent. 

Salt-cake  possesses  certain  other  properties  that  make  it 
somewhat  troublesome  to  deal  with  as  a  glass-making 
material.  Thus,  on  prolonged  exposure,  particularly  to 
moist  air,  the  powdered  salt-cake  absorbs  moisture  from 
the  atmosphere  and  undergoes  partial  conversion  into  the 
crystalline  form  of  "  Glauber's  Salt,"  a  process  which 
results  in  the  formation  of  exceedingly  hard  masses. 
Ground  salt-cake,  therefore,  cannot  be  stored  for  any 
length  of  time  without  incurring  the  necessity  of  re- 
grinding,  and  this  accretive  action  even  comes  into  play 
when  mixtures  of  glass-making  materials,  containing  salt- 
cake  as  one  ingredient,  are  stored.  In  practice,  therefore, 
salt-cake  can  only  be  ground  as  it  is  wanted,  and  its  physical 
properties  make  it  difficult  to  grind  it  at  all  fine,  while  the 
dust  arising  from  this  process  is  peculiarly  irritating, 
although  not  seriously  injurious  to  health. 

Potash  is  utilised  in  glass-making  almost  entirely  in  the 
form  of  carbonate,  generally  called  "  pearl-ash."  Origin- 
ally derived  from  the  ashes  of  wood  and  other  land  plants, 
this  substance  is  now  manufactured  by  processes  similar  to 
those  described  in  the  case  of  soda,  the  raw  material  being 
potassium  chloride  derived  from  natural  deposits  such  as 


44  GLASS  MANUFACTUEE. 

those  at  Stassfurth.  The  pearl-ash  thus  commercially 
obtainable  is  a  fairly  pure  substance,  but  its  use  is  com- 
plicated by  the  fact  that  it  is  strongly  hygroscopic  and 
rapidly  absorbs  water  from  the  atmosphere.  Where  it  is 
desired  to  produce  potash  glasses  of  constant  composition, 
frequent  analytical  determinations  of  the  moisture  contents 
of  the  pearl-ash  are  necessary,  and  the  composition  of  the 
glass  mixture  requires  adjustment  in  accordance  with  the 
results  of  these  determinations. 

The  alkalies  are  also  introduced  into  glass  in  the  form  of 
nitrates  (potassium  nitrate,  or  saltpetre,  and  sodium  nitrate, 
or  nitre) ;  but  although  these  substances  act  as  sources  of 
alkali  in  the  glass,  they  are  employed  essentially  for  the 
sake  of  their  oxygen  contents.  Such  oxidising  agents  are 
not,  of  course,  added  to  glass  mixtures  containing  sulphates 
and  carbon,  but  are  employed  to  purify  the  mixtures  con- 
taining alkali  carbonates,  and  more  especially  to  oxidise  the 
flint  glasses.  Since  these  substances  are  only  introduced  into 
glass  in  small  quantities  their  extreme  purity  is  not  of  such 
great  importance  to  the  glass-maker,  and  the  ordinary 
"  refined  "  qualities  of  both  nitrates  are  found  amply  pure 
enough  to  answer  the  highest  requirements. 

A  certain  number  of  natural  minerals  which  contain  an 
appreciable  quantity  of  alkali  are  sometimes  utilised  as  raw 
materials  for  glass  manufacture.  The  most  important  of 
these  are  the  minerals  of  the  felspar  class  already  referred 
to.  These,  however,  contain  a  considerable  proportion  of 
alumina,  while  all  but  the  purest  varieties  also  contain 
more  or  less  considerable  quantities  of  iron.  Some  glass- 
makers  regard  alumina  as  an  undesirable  constituent,  while 
others  take  the  opposite  view,  and  upon  this  view  their  use 


THE  RAW  MATEEIALS  OF   GLASS  MANUFACTURE.    45 

of  felspathic  minerals  will  depend.  For  the  cheaper 
varieties  of  glass,  however,  such  as  bottle  glass,  felspathic 
minerals  and  rocks,  such  as  granite  and  basalt,  are  freely 
used  as  raw  materials.  Another  mineral  in  which  both 
alkali  and  alumina  are  found  is  cryolite.  This  mineral 
is  a  double  fluoride  of  soda  and  alumina,  whose  properties 
are  particularly  valuable  in  the  production  of  opal  and 
opalescent  glasses.  As  a  mere  source  of  alkali,  however, 
cryolite  is  much  too  expensive. 

(3)  Sources  of  Bases  other  than  Alkalies. — The  most  impor- 
tant of  these  are  lime  and  lead  oxide,  the  former  being  re- 
quired for  the  production  of  all  varieties  of  plate  and  sheet- 
glass,  as  well  as  for  bottles  and  a  large  proportion  of  pressed 
and  blown  glass,  while  lead  is  an  essential  ingredient  of  all 
flint  glass.  The  only  other  base  having  any  considerable 
commercial  importance  in  connection  with  glass-making 
is  barium  oxide,  while  oxide  of  zinc,  magnesia,  and  a  few 
other  substances  are  used  in  the  manufacture  of  special 
glasses  for  scientific,  optical  or  technical  purposes,  where 
glass  of  special  properties  is  required.  The  metallic  oxides 
\vhich  are  used  for  the  production  of  coloured  glass  are,  of 
course,  also  basic  bodies.  These  will  be  treated  in  connection 
with  coloured  glasses,  with  the  exception  of  manganese 
dioxide,  which  is  used  in  large  quantities  in  the  manufacture 
of  many  ordinary  "  white  "  glasses. 

Calcium  Oxide  (lime)  is  generally  introduced  into  glass 
mixtures  in  the  form  of  either  the  carbonate  or  the  hydrated 
oxide  (slaked  lime).  The  carbonate  may  be  derived  either 
from  natural  sources,  or  it  may  be  of  chemical  origin,  while 
the  hydrate  is  always  obtained  by  the  calcination  of  the 
carbonate,  followed  by  "  slaking"  the  lime  thus  produced. 


46  GLASS  MANUFACTUBE. 

Natural  calcium  carbonate  occurs  in  great  quantities  in  the 
form  of  chalk  and  limestone  rocks.  Both  varieties  are  used 
for  glass-making.  Chalk  is  a  soft  friable  material  which 
is  apt  to  clog  during  the  grinding  operations,  particularly 
as  the  natural  product  is  generally  somewhat  moist.  As 
regards  the  greater  part  of  its  mass,  chalk  is  often  found 
in  a  state  of  great  purity,  but  it  is  frequently  contaminated 
by  the  presence  of  scattered  masses  of  flint.  Chemically 
this  impurity  is  not  very  objectionable  to  the  glass-maker, 
since  it  merely  introduces  a  small  proportion  of  silica  whose 
presence  need  scarcely  be  allowed  for  in  laying  down  the 
mixture.  On  the  other  hand,  if  any  fragments  of  flint 
remain  in  the  mixture  when  put  into  the  furnace,  they 
prove  very  refractory,  and  are  apt  to  be  found  as  opaque 
enclosures  in  the  finished  glass.  Natural  limestone  can 
also  be  obtained  in  great  purity  in  many  parts  of  the 
world.  It  is  generally  a  hard  and  rather  brittle  rock  that 
can  be  readily  ground  to  powder  of  the  requisite  degree  of 
fineness.  Flint  concretions  are  not  so  frequently  found  in 
this  material,  but,  on  the  other  hand,  it  is  often  contaminated 
with  magnesia  and  iron.  The  former  ingredient,  when 
present  in  small  quantities,  tends  to  make  the  glass  hard 
and  viscous,  so  that  limestone  of  the  lowest  possible 
magnesia  content  should  be  used,  especially  for  the  harder 
kinds  of  glass,  such  as  plate  and  sheet-glass,  etc.  The  iron 
contents  of  the  limestone  used  must  also  be  low  where  a 
white  glass  is  required ;  but  since  a  smaller  quantity  of 
limestone  is  used  for  a  given  weight  of  glass  produced  than 
the  quantity  of  sand  used  for  the  same  purpose,  the 
presence  of  a  somewhat  higher  percentage  of  iron  is 
permissible  in  the  limestone  as  compared  with  the  sand ; 


THE  RAW  MATERIALS  OF   GLASS  MANUFACTURE.    47 

for  the  better  varieties  of  glass,  however,  the  iron  should 
not  exceed  0'3  per  cent,  of  the  limestone. 

Slaked  lime  is  sometimes  used  as  the  source  of  lime  for 
special  glasses  where  the  process  of  manufacture  renders 
it  desirable  to  avoid  the  evolution  of  carbonic  acid  gas 
which  takes  place  when  the  carbonate  is  heated  and 
attacked  by  silica.  When  slaked  lime  is  used  only  the 
water  vapour  of  the  hydrate  is  driven  off,  and  this  occurs 
at  a  much  lower  temperature.  For  the  production  of 
slaked  lime,  an  adequately  pure  form  of  limestone,  prefer- 
ably in  the  form  of  large  lumps,  is  burnt  in  a  kiln  until 
the  carbonic  acid  is  entirely  driven  off;  after  cooling,  the 
lime  so  formed  is  slaked  by  hand.  The  product  so  obtained 
is,  however,  apt  to  vary  both  as  regards  contents  of 
moisture  and  carbonic  acid,  which  latter  is  readily  absorbed 
from  the  atmosphere ;  the  use  of  this  material,  therefore, 
requires  frequent  analytical  determinations  of  the  lime 
contents  and  corresponding  adjustments  of  the  mixture  if 
constant  results  are  required. 

It  is  possible  to  introduce  lime  into  glass  mixtures  in  the 
form  of  gypsum  or  calcium  sulphate,  but  the  decomposition 
of  this  compound,  like  that  of  sodium  sulphate,  requires 
the  intervention  of  a  reducing  agent  such  as  carbon,  and 
the  difficulties  arising  from  this  source  in  connection  with 
the  use  of  salt-cake  are  still  further  increased  in  the  case 
of  the  calcium  compound.  Since  limestones  of  considerable 
purity  are  more  or  less  plentiful  in  many  districts,  the 
commercial  value  of  calcium  sulphate  for  glass-making  is 
probably  slight. 

The  Compounds  of  Barium  may  best  be  dealt  with  at  this 
stage,  since  they  are  chemically  so  closely  allied  to  the 


48  GLASS  MANUFACTURE. 

compounds  of  lime  just  described.  Barium  occurs  in 
nature  in  considerable  quantities  in  the  minerals  known 
as  barytes  (heavy  spar)  and  witherite  respectively.  The 
former  is  essentially  sulphate  of  barium,  while  the 
latter  is  a  carbonate  of  barium.  The  use  of  the  sulphate 
meets  with  the  same  objection  here  as  in  the  case  of 
calcium  sulphate  discussed  above,  except  that  the  barium 
compound  is  much  more  easily  reduced  and  decomposed 
than  the  lime  compound.  The  natural  mineral  witherite 
is  used  to  a  considerable  extent  in  the  production  of  barium 
glasses,  and  these  have  been  found  capable  of  replacing 
lead  glasses  for  certain  purposes.  On  the  other  hand,  for 
the  best  kinds  of  barium  glasses,  viz.,  those  required  for 
optical  purposes,  the  element  is  introduced  in  the  form  of 
artificially  prepared  salts.  Of  these  the  most  important  is 
the  carbonate,  commercially  described  as  "  precipitated 
carbonate  of  barium  "  ;  this  precipitated  compound,  how- 
ever, does  not  ordinarily  correspond  to  the  chemically  pure 
substance,  but  contains  more  or  less  considerable  quantities 
of  sulphur  compounds.  The  question  whether  these* 
impurities  are  or  are  not  objectionable  can  only  be  deter- 
mined for  each  particular  case,  since  much  depends  upon 
the  special  character  of  the  glass  to  be  produced.  Both  the 
nitrate  and  the  hydrate  of  barium  are  commercially  avail- 
able, but  they  are  very  costly  ingredients  for  use  in  the 
production  of  even  the  most  expensive  kinds  of  glass  ;  these 
substances  are,  however,  obtainable  in  a  state  of  considerable 
purity,  although  the  hydrate  has  the  inconvenient  property 
of  rapidly  absorbing  carbonic  acid  from  the  atmosphere, 
thus  becoming  converted  into  the  carbonate. 

Magnesia  is  another  glass-forming  base  that  is  closely 


THE  RAW  MATERIALS   OF  GLASS  MANUFACTURE.     49 

related,  chemically,  to  calcium  and  barium.  This  element 
is  usually  introduced  into  glass  mixtures  in  the  form  of 
either  the  carbonate  or  the  oxide.  The  carbonate  occurs  in 
nature  in  a  more  or  less  pure  state  in  the  form  of  magnesite, 
and  by  calcination,  the  oxide  is  obtained.  The  natural 
mineral  and  its  product  are,  of  course,  by  far  the  cheapest 
sources  of  magnesia,  but  as  the  element  is  only  used  in 
comparatively  small  quantities,  the  artificial  precipitated 
carbonate  or  calcined  magnesia  are  frequently  preferred. 
Magnesia  is  only  introduced  intentionally  in  notable 
quantities  in  special  glasses  where  the  properties  it  confers 
are  of  special  value  ;  in  ordinary  lime  glasses  this  element, 
as  has  already  been  mentioned,  is  to  be  regarded  as  an 
undesirable  impurity. 

Zinc  oxide  lies,  chemically,  between  the  bases  already 
discussed  on  the  one  hand,  and  lead  oxide  on  the  other. 
This  element  is  only  introduced  into  special  optical  glasses, 
a  special  "  zinc  crown"  having  found  some  application. 
Chemically  prepared  zinc  oxide  is  almost  the  only  form  in 
which  the  element  is  used,  but  the  very  volatile  character 
of  this  substance  must  be  borne  in  mind  when  it  is 
introduced  into  glass  mixtures. 

Lead  is  one  of  the  most  widely-used  ingredients  of  glass ; 
the  glasses  containing  this  substance  in  notable  quantity 
are  all  characterised  to  a  greater  or  less  degree  by  similar 
properties,  such  as  considerable  density  and  high  refractive 
power,  and  are  classed  together  under  the  name  "  flint 
glasses."  Lead  is  now  almost  universally  introduced  into 
glass  mixtures  in  the  form  of  red  lead,  although  the  other 
oxides  of  lead  might  be  employed  almost  equally  well.  Eed 
lead  is  a  mixture  of  two  oxides  of  lead  (PbO  and  Pb203)  in 

G.M.  E 


50  GLASS   MANUFACTURE. 

approximately  such  proportions  as  to  correspond  to  the 
formula  Pb304.  It  is  prepared  by  the  roasting  of  metallic 
lead  in  suitable  furnaces,  where  the  molten  lead  is  exposed 
to  currents  of  hot  air.  The  product  is  obtainable  in  con- 
siderable purity,  very  small  proportions  of  silica,  derived 
from  the  furnace  bed,  and  of  iron  derived  from  the  tools 
with  which  the  lead  is  handled,  being  the  principal  foreign 
substances  found  in  good  red  lead.  Silver  would  be  an 
objectionable  impurity,  but  owing  to  the  modern  perfect 
methods  of  de-silvering  lead,  that  element  is  rarely  found 
in  lead  products.  Analytical  control  of  red  lead  as  used  in 
the  glass  mixtures,  and  consequent  adjustments  of  the 
mixture,  are,  however,  necessary  where  exact  constancy  in 
the  glass  produced  is  desired.  The  reason  for  this  necessity 
lies  in  the  fact  that  the  oxygen  content,  and  therefore  the 
lead-oxide  (PbO)  content,  varies  decidedly  from  batch  to 
batch,  while  the  material  as  actually  delivered  and  used 
frequently  contains  notable  proportions  of  moisture. 

A  word  should  perhaps  be  said  here  as  to  methods  of 
handling  red  lead  on  account  of  the  injurious  effects  which 
the  inhalation  of  lead  dust  produces  upon  the  workmen 
exposed  to  it.  For  glass-making  purposes  it  is  not  feasible 
to  adopt  the  method  adopted  by  potters  of  first  "  fritting  " 
the  lead  and  thus  rendering  it  comparatively  insoluble  and 
innocuous  ;  even  if  this  were  done,  the  difficulty  would  only 
be  moved  one  step  further  back,  and  would  have  to  be  over- 
come by  those  who  undertook  the  preparation  of  the  frit. 
The  proper  solution  of  the  problem,  in  the  writer's  opinion, 
is  to  be  found  in  properly  preventing  the  formation  of  lead 
dust,  or  at  all  events  in  protecting  the  workmen  from  the 
risk  of  inhaling  it.  Where  only  small  quantities  of  lead 


THE  RAW  MATERIALS  OF   GLASS  MANUFACTURE.     51 

glass  are  made,  and  therefore  only  small  quantities  of  lead 
are  handled  and  mixed  at  a  time,  it  is  no  doubt  sufficient  to 
provide  the  workmen  engaged  on  this  task  with  some 
efficient  form  of  respirator  to  be  worn  during  the  whole  of 
the  time  that  they  are  engaged  on  such  work,  and  to  take 
the  further  precautions  necessary — by  way  of  cleanliness 
and  the  provision  of  proper  mess-rooms — to  avoid  any  risk 
of  lead  dust  either  directly  or  indirectly  contaminating 
their  food.  Where,  however,  large  quantities  of  flint-glass 
are  made  every  day,  it  is  possible  and  proper  to  make 
more  perfect  arrangements  for  the  mechanical  handling  and 
mixing  of  the  lead  with  the  other  ingredients  by  the  provision 
of  suitable  mixing  and  transporting  machinery,  so  arranged 
as  to  be  dust-tight.  It  is  only  fair  to  state,  however,  that 
partly  under  their  own  initiative,  partly  under  pressure 
from  the  authorities,  glass  makers  in  this  country  are 
complying  with  these  requirements  in  an  adequate  manner. 
Aluminium. — There  are  several  varieties  of  glass  into 
which  alumina  enters  in  notable  quantities,  the  principal 
examples  being  certain  optical  and  many  opal  glasses,  while 
most  ordinary  glasses  contain  this  substance  in  greater  or 
less  degree.  In  the  latter,  the  alumina  is  derived  by  the 
inevitable  processes  of  solution,  from  the  fire-clay  vessels  or 
walls  within  which  the  molten  glass  is  contained,  while  in 
some  cases  the  element  is  intentionally  introduced  in  small 
proportions  (about  2  per  cent,  to  3  per  cent,  of  A1203)  by  the 
use  of  felspar  as  an  ingredient  of  the  mixture.  Where 
larger  proportions  of  alumina  are  required,  the  substance  is 
introduced  in  the  form  of  the  hydrate,  which  is  obtainable 
commercially  in  a  state  of  almost  chemical  purity,  but  of 
course  at  a  correspondingly  high  cost.  In  opal  glasses 

E  2 


52  GLASS  MANUFACTUEE. 

alumina  is  derived  partly  or  wholly  from  felspars,  or  in 
some  cases  from  the  use  of  the  mineral  cryolite.  This  is  a 
double  fluoride  of  aluminium  and  sodium  which  is  found  in 
great  natural  masses,  chiefly  in  Greenland.  Owing  to  the 
high  price  of  this  mineral,  however,  artificial  substitutes  of 
nearly  identical  composition  and  properties  have  been 
introduced  and  are  used  successfully  in  the  glass  and 
enamelling  industries. 

Manganese. — Although  the  oxides  of  this  element  really 
belong  to  the  class  of  colouring  compounds,  they  are  so 
widely  used  in  the  manufacture  of  ordinary  "  white  "  glasses 
that  it  is  desirable  to  deal  with  them  here.  The  element 
manganese  is  most  usually  introduced  into  glass  mixtures 
in  the  form  of  the  per-oxide  (MnC^),  although  the  lower 
oxide  (MnsOi)  can  also  be  used.  The  material  ordinarily 
used  is  the  natural  manganese  ore,  mined  chiefly  in  Eussia ; 
the  purest  forms  of  this  ore  consist  almost  entirely  of  the 
per-oxide,  but  "  brown  "  ores,  containing  more  or  less  of 
the  lower  oxide,  are  also  used  with  success.  These  ores 
always  contain'  small  amounts  of  iron  and  silica,  but 
provided  the  iron  is  not  present  in  any  considerable  quantity, 
the  value  of  the  ore  is  measured  by  the  percentage  of 
manganese  which  it  contains.  The  colouring  and  "  decolour- 
ising "  action  of  manganese  will  be  discussed  in  a  later 
chapter.  Certain  other  substances,  which  have  been 
suggested  as  either  substitutes  for,  or  improvements  upon, 
manganese  for  this  purpose  need  only  be  mentioned  here, 
viz.,  nickel,  selenium  and  gold. 

Arsenic  is  another  substance  frequently  introduced  into 
"  white  "  glass  mixtures.  This  element  is  universally 
introduced  in  the  form  of  the  white  arsenic  of  commerce 


THE  EAW  MATEEIALS   OF   GLASS  MANUFACTUEE.     53 

(i.e.,  arsenious  acid,  As203)  which  is  obtained  in  a  pure  form 
by  a  process  of  sublimation.  Owing  to  the  very  poisonous 
nature  of  this  material,  special  precautions  must  be  taken 
in  its  use  for  glass-making  purposes  to  avoid  all  risk  of 
poisoning. 

Carbon. — As  has  already  been  indicated,  an  admixture 
of  carbon  in  some  suitable  form  is  essential  in  the  case  of 
certain  glass  mixtures.  The  carbon  for  this  purpose  may 
be  used  in  the  form  of  either  charcoal,  coke,  or  anthracite 
coal.  Of  these,  charcoal  is  undoubtedly  the  purest  form  of 
carbon,  but  it  is  excessively  expensive  in  this  country. 
Coke  varies  very  much  in  quality  according  to  the  coal 
from  which  it  has  been  produced,  but  it  always  contains 
notable  proportions  of  ash  rich  in  iron,  and  also  some 
sulphur.  Anthracite  coal  can  be  obtained  in  a  very  pure 
form,  containing  considerably  less  ash  than  that  found  in 
most  kinds  of  coke,  and  this  is  therefore  probably  the  most 
convenient  form  of  carbon  for  this  purpose. 


CHAPTEK    IV. 

CRUCIBLES    AND    FURNACES    FOR    THE    FUSION    OF    GLASS. 

FOR  the  successful  production  of  substances  which  are 
formed  by  a  process  of  fusion,  the  use  of  refractory 
materials  of  a  proper  kind  is  of  great  importance.  In  the 
production  of  glass  the  double  difficulty  has  to  be  overcome 
of  finding  substances  capable  of  being  formed  into  furnaces 
and  crucibles  which  shall  not  only  resist  the  softening  and 
melting  action  of  the  furnace  heat  for  long  periods  of  time, 
but  shall  also  resist  the  dissolving  action  of  the  molten 
glass  itself.  The  refractory  materials  employed  in  connec- 
tion with  glass-making  thus  fall  into  two  distinct  groups, 
members  of  one  group  being  those  which  meet  both  of  the 
above  requirements  and  can  therefore  be  used  in  positions 
exposed  to  direct  contact  with  molten  glass,  while  members 
of  the  second  group  are  materials  which  resist  the  action 
of  the  heat  and  flame  gases  but  cannot  resist  the  dissolving 
effect  of  the  glass  itself;  these,  of  course,  can  only  be 
placed  where  molten  glass  is  not  liable  to  touch  them.  We 
shall  deal  with  the  former  group  first. 

Those  portions  of  glass-melting  plant  which  come  into 
contact  with  molten  glass  are  almost  universally  made  of 
some  form  of  fire-clay.  To  discuss  in  detail  the  composi- 
tion and  properties  of  the  varieties  of  fire-clay  best  suited 


CRUCIBLES  AND  FURNACES  FOR  FUSION  OF  GLASS.    55 

to  this  purpose  would  exceed  the  entire  limits  of  this  book, 
so  that  only  a  few  leading  principles  can  be  stated.  Taking 
first  the  clays  intended  for  the  production  of  crucibles  or 
"  pots,"  we  find  that  for  the  purposes  of  the  production  of 
such  objects  the  prepared  clay  must  possess  a  certain 
degree  of  plasticity  while  damp  and  a  considerable  degree 
of  strength  when  dried.  The  dried  and  burnt  material 
must  be  so  refractory  as  to  resist  the  high  temperatures 
used  in  glass-melting  without  undergoing  fusion  or  even 
serious  softening.  Clays  of  various  composition  and 
physical  nature  also  differ  very  widely  in  their  power  of 
resisting  the  chemical  attack  of  molten  glass  ;  all  clays  are 
more  or  less  dissolved  under  these  circumstances,  but  not 
only  the  rate,  but  also  the  manner,  of  dissolution  is  of 
importance,  so  that  frequently  a  clay  which  dissolves 
rapidly  but  uniformly  is  preferred  to  one  which  dissolves 
more  slowly  but  in  such  an  irregular  manner  as  to  throw 
off  particles  of  undissolved  material  which  contaminate  the 
glass  in  the  form  of  opaque  enclosures  or  "  stones.'7  It  is 
also  to  be  noted  that  the  best  results  in  this  direction  can 
only  be  obtained  by  careful  adaptation  of  the  clay  employed 
to  the  particular  kind  of  glass  which  is  to  be  melted  in  the 
crucibles  in  question.  In  England  this  question  has  not 
received  the  amount  of  attention  it  deserves,  but  in 
Germany  and  America  the  available  fire-clays  of  the 
country  have  been  systematically  studied  and  exploited. 
As  a  result  the  glass-maker  has  at  his  disposal  a  large 
selection  of  materials  of  accurately  known  physical  and 
chemical  properties.  By  carefully  correlating  these  with 
the  performance  of  his  "  pots  "  in  the  furnaces,  the  manu- 
facturer is  able  to  select  the  most  suitable  material,  and  is, 


56 


GLASS  MANUFACTUBE. 


moreover,  in  a  position  to  know  in  what  direction  to  look 
for  improvement  or  for  replacement  if  the  supply  of  a 
satisfactory  brand  should  cease. 

We  may  now  follow  briefly  the  process  of  manufacture  of 
a  fire-clay  pot  or  crucible.  The  size  and  shape  of  the 
crucible  will  depend  upon  the  particular  purpose  for  which 
it  is  intended.  Crucibles  varying  in  capacity  from  4  cwt. 
to  2J  tons  of  glass  are  used  for  various  kinds  of  glass,  but 
the  more  usual  sizes  lie  between  30  in.  and  50  in.  in 
diameter.  For  many  kinds  of  glass  the  shape  of  the  pot  is 


FIG.  1. — Open  "  pot "  or  crucible 
for  glass  melting. 


FIG.  2.— Covered  pot  for  glass 
melting,  as  used  for  flint 
glass  and  optical  glass. 


simply  that  of  an  open  basin,  circular  or  oval  in  plan  and 
larger  in  diameter  at  the  brim  than  at  the  base  (Fig.  1), 
but  for  the  production  of  flint  glass,  and  of  other  glasses 
which  are  to  be  protected  from  contact  with  the  flame  and 
gases  of  the  furnace,  so-called  "covered"  pots  are  used. 
In  these  the  basin — here  of  a  more  nearly  cylindrical 
shape — is  covered  over  by  a  dome,  and  access  is  allowed 
only  by  a  relatively  small  hooded  opening  (Fig.  2). 
Covered  pots  are  built  up  on  wooden  moulds,  which  are 
made  collapsible,  and  are  removed  before  the  drying  of  the 
pot  is  begun. 

The  material  for  pot-making  is  first  prepared  with  great 


CRUCIBLES  AND  FURNACES  FOR  FUSION  OF  GLASS.     57 

care.  The  proper  variety  of  clay  having  been  selected,  it 
is  ground  to  a  fine  powder  in  suitable  mills  and  carefully 
sieved  ;  with  this  fine  clay  powder  is  mixed,  in  accurately 
determined  proportions,  a  quantity  of  crushed  burnt  fire- 
clay. In  some  works  this  burnt  material  is  obtained  by 
simply  grinding  up  fragments  of  old  used  pots,  but  the 
better  practice  is  to  burn  specially-selected  fire-clay  sepa- 
rately for  this  purpose.  The  quantity  of  such  burnt 
material  added  to  the  mixture  depends  upon  the  chemical 
nature  and  especially  on  the  plasticity  of  the  virgin  clay 
employed  ;  with  so-called  "  fat  "  or  very  plastic  clays  up  to 
50  per  cent,  of  burnt  material  is  added,  but  with  the  leaner 
clays,  such  as  those  of  the  Stourbridge  district  in  England, 
very  much  smaller  proportions  are  used.  The  object  of 
this  addition  of  burnt  material  is  to  facilitate  the  safe  dry- 
ing of  the  finished  pots  and  to  diminish — by  dilution — the 
total  amount  of  contraction  which  takes  place  both  when 
plastic  clay  is  allowed  to  dry,  and  further  when  the  dry 
mass  is  subsequently  burnt;  the  burnt  material  or 
"charnotte,"  having  already  undergone  these  shrinking 
processes,  acts  both  as  a  neutral  diluent  and  also  as  a 
skeleton  strengthening  the  whole  mass  and  reducing  the 
tendency  to  form  cracks. 

The  virgin  clay  and  chamotte  having  been  intimately 
mixed,  the  whole  mass  is  "  wet  up  "  by  the  addition  of  a 
proper  proportion  of  water  and  prolonged  and  vigorous 
kneading,  usually  in  a  suitable  pug  mill.  The  mass  leaves 
this  mill  as  a  fairly  stiff,  plastic  dough,  but  the  full  tough- 
ness and  plasticity  of  such  clay  mixtures  can  only  be 
developed  by  prolonged  storage  of  the  damp  mass.  In  the 
next  stage  of  the  process,  the  plastic  clay  is  passed  to  the 


58  GLASS  MANUFACTUBE. 

"  pot  maker  "  in  the  form  of  thick  rolls,  and  with  these  he 
gradually  builds  up  the  pots  or  crucibles  from  day  to  day, 
allowing  the  lowest  parts  to  dry  sufficiently  to  enable  them 
to  bear  the  weight  of  the  upper  parts  without  giving  way. 
The  building  of  large  pots  in  this  way  occupies  several 
weeks,  and  during  this  time  the  premature  drying  of  any 
part  of  the  pot  must  be  carefully  avoided.  After  the  com- 
pletion of  the  pot,  drying  is  allowed  to  take  place,  slowly 
at  first,  but  more  vigorously  after  a  time  when  the  risk  of 
cracking  is  smaller ;  when  it  is  taken  into  use,  the  pot  is 
usually  many  months  old  and  is  thoroughly  air-dry.  The 
clay,  however,  is  still  hydrated,  i.e.,  contains  chemically 
combined  water,  and  this  is  only  expelled  during  the  early 
stages  of  the  burning  process.  This  process  is  carried  out 
in  smaller  furnaces  or  kilns  placed  near  the  melting 
furnaces.  In  these  the  pot  or  pots  are  exposed  to  a  very 
gradually  increasing  temperature  until  a  bright  red  heat  is 
finally  attained.  This  is  a  delicate  process  in  which  great 
care  is  required  to  secure  gradual  and  uniform  heating, 
especially  during  the  earlier  stages,  otherwise  the  pots  are 
apt  to  crack  and  become  useless.  Finally,  when  a  bright 
red  heat  has  been  maintained  for  at  least  a  day,  the  pots 
are  ready  to  be  placed  in  the  furnace,  and  this  is  ordinarily 
done  while  both  pots  and  furnace  are  at  a  red  heat,  the 
pots  never  being  allowed  to  cool  down  again  once  they  have 
been  burnt. 

Fire-clay  is  also  used  in  the  manufacture  of  bricks  and 
blocks  of  various  sizes  required  for  the  construction  of  glass- 
melting  furnaces.  Here  fire-clay  is  only  used  in  positions 
where  contact  with  molten  glass  is  expected,  as  in  the  walls 
of  the  basin  or  tank  proper  in  "  tank  "  furnaces,  or  at  a 


CEUCTBLES  AND  FUENAOES  FOB,  FUSION  OF  GLASS.    59 

level  below  that  of  the  pot  or  crucible  in  pot  furnaces  ;  in 
the  latter  position  leakage  of  glass  from  broken  pots  or 
overflow  being  liable  to  result  in  an  accumulation  of 
molten  glass  on  the  floors  or  walls  of  the  furnace  and 
passages.  The  fire-bricks  used  in  these  latter  positions  are 
usually  of  a  much  poorer  quality  of  fire-clay  than  that  used 
for  the  manufacture  of  pots,  and  this  is  justified  in  so  far  as 
certain  of  the  requirements  that  apply  to  crucibles  do  not 
apply  here — but  on  the  other  hand  the  use  of  more 
refractory  bricks  would  result  in  a  longer  life  for  the 
furnace.  Such  bricks,  it  should  be  noted,  are  not  laid  in 
mortar  when  used  for  furnace  construction,  but  are  set  in 
a  thin  paste  of  fire-clay  in  water,  and  these  joints  are  kept 
as  thin  as  possible.  The  part  of  the  furnace  known  as  the 
"  siege"  (French  "  siege"),  i.e.,  the  floor  of  the  furnace 
upon  which  the  pots  are  placed,  is  usually  built  of  very 
large  blocks  of  fire-clay,  made  of  coarse  materials  calculated 
to  give  great  strength.  At  or  near  the  points  where  the 
flame  enters  the  furnace,  these  blocks  rapidly  wear  away, 
partly  by  melting  but  chiefly  by  a  process  of  abrasion,  for 
it  seems  that  a  rapidly  moving  flame  has  an  abrading 
action  of  a  very  marked  kind. 

The  actual  tanks  or  basins  which  contain  the  molten 
glass  in  tank  furnaces  are  also  built  of  large  blocks  of  fire- 
clay, but  these  are  made  of  the  best  procurable  materials, 
and  should  receive  at  least  as  much  care  in  every  respect 
as  crucibles ;  it  is  true  that  their  shape  and  size  gives  them 
greater  strength,  but  on  the  other  hand  these  blocks  are 
expected  to  resist  the  contact  of  molten  glass  for  very  much 
longer  periods  of  time  than  the  average  crucible.  To 
understand  the  requirements  for  tank-blocks  it  is  necessary 


60  GLASS  MANUFACTURE. 

to  anticipate  the  next  section  to  the  extent  of  stating  that 
in  tank  furnaces  the  glass  is  contained,  during  melting, 
refining  and  working,  in  a  basin  built  up  of  large  blocks. 
These  blocks  are  not  cemented  together  in  any  way,  but 
are  built  up  "  dry  "  and  are  supported  on  the  outside  by  a 
system  of  iron  bars  and  rods.  The  molten  glass  penetrates 
between  the  blocks  to  a  certain  extent,  but  as  the  outside  of 
all  such  blocks  is  intentionally  kept  as  cold  as  possible  the 
glass  rapidly  stiffens  as  it  penetrates  further  into  these 
interstices,  and  this  stiffened  glass  effectually  binds  the 
blocks  together  and  prevents  all  leakage.  It  will  thus  be 
seen  that  the  blocks  are  exposed  to  the  full  heat  of  the 
furnace  and  to  the  corroding  action  of  the  glass  on  the 
inner  side,  but  are  kept  cold  on  the  outer  side.  As  this 
state  of  affairs  tends  to  produce  cracks,  these  blocks  are 
necessarily  made  of  rather  coarse  material.  On  the  other 
hand,  the  material  of  a  block  never  gets  so  hot  as  the  wall 
of  a  crucible,  which  is  heated  from  both  sides,  so  that 
extreme  refractoriness  is  not  so  essential. 

It  is  impossible,  within  the  limits  of  this  chapter,  to  go 
into  the  details  of  the  choice  of  materials  for  tank-blocks ; 
it  is  a  subject  upon  which  no  finally  satisfactory  conclusion 
has  yet  been  reached,  and  what  has  been  said  above  will 
suffice  to  show  the  nature  of  the  considerations  upon  which 
such  choice  must  be  based. 

We  now  turn  to  the  second  class  of  refractory  materials 
used  in  the  construction  of  glass-melting  furnaces,  viz., 
those  which  are  so  placed  as  not  to  come  into  contact  with 
molten  glass.  Here  mechanical  strength  and  refractoriness 
are  almost  the  only  considerations,  but  in  the  roof-vaults  or 
"  crowns  "  of  tank  furnaces  and  also  of  furnaces  in  which 


CEUOIBLES  AND  FUENACES  FOE  FUSION  OF  GLASS.    61 

glass  is  melted  in  open  pots,  there  is  the  further  considera- 
tion that  the  material  of  the  bricks  used  shall  not  contain 
notable  quantities  of  any  colouring  oxide,  since  small 
flakes,  etc.,  are  apt  to  drop  down  into  the  molten  glass,  and 
would  thus  be  liable  to  cause  serious  discolouration.  Such 
a  material  as  chrome-ore  brick  is  therefore  excluded.  As  a 
matter  of  fact,  some  form  of  "  silica  brick  "  is  in  universal 
use.  Bricks  of  this  material,  otherwise  known  as  "  Dinas 
bricks"  from  the  place  of  their  first  origin,  in  Wales,  con- 
sist of  about  98  per  cent,  of  silica  (Si02).  Pure  silica 
cannot  be  baked  or  burnt  into  coherent  bricks  entirely  by 
itself,  since  it  possesses  neither  plasticity  when  wet  nor  any 
binding  power  when  burnt,  but  an  admixture  of  about 
2  per  cent,  of  lime  and  a  little  alumina  makes  it  possible 
first  to  mould  the  bricks  when  wet  and  then  to  burn  them 
so  as  to  form  fairly  strong,  coherent  blocks.  These  are  of 
amply  adequate  refractoriness  for  the  highest  temperatures 
that  can  be  attained  in  industrial  gas-fired  furnaces,  and 
their  mechanical  strength  is  sufficient  to  make  it  possible 
to  build  vaults  of  considerable  span,  but  on  the  other  hand 
this  material  requires  very  gradual  heating  and  constant 
watching  while  the  temperature  is  rising  or  falling  to  any 
considerable  extent ;  the  reason  for  this  difficulty  lies  in  the 
fact  that  silica  bricks  swell  very  markedly  during  heating, 
so  that  unless  a  vault  built  of  this  material  is  given  room 
to  spread  somewhat,  it  will  rise  seriously  and  may  even 
break  up  completely.  This  risk  is  avoided  by  gradually 
slackening  the  tie-bolts  that  hold  the  vault  together,  and 
correspondingly  "  taking  up  the  slack  "  as  the  vault  cools 
when  the  furnace  is  let  out.  Sudden  local  heat  also  has  a 
Disastrous  effect  on  this  material,  producing  serious  flaking. 


62  GLASS  MANUFACTURE. 

For  positions  where  intense  heat  is  to  be  borne,  and 
at  the  same  time  mechanical  strength  is  required,  silica 
brick  is  a  most  valuable  material,  but  owing  to  its  chemical 
composition  it  is  rapidly  attacked  by  molten  glass  or  by  any 
material  containing  a  notable  proportion  of  basic  con- 
stituents, so  that  the  silica  bricks  can  only  be  employed 
out  of  contact  with  glass. 

We  now  turn  to  consider,  very  briefly,  the  general  design 
and  arrangement  of  some  typical  glass-melting  furnaces. 
The  oldest  and  simplest  form  of  furnace  is,  in  effect,  simply 
a  box  built  of  fire-brick,  in  the  centre  of  which  stands  the 
crucible,  while  a  fire  of  wood  or  coal  is  placed  upon  either 
side.  To  attain  any  great  degree  of  heat  by  such  means, 
however,  the  size  of  the  box  or  chamber  and  especially  of 
the  grates  in  which  the  fires  are  maintained  must  be 
properly  proportioned  both  to  the  dimensions  of  the  crucible 
and  to  each  other.  The  grates  are  generally  wide  and 
deep,  while  draught  is  provided  by  means  of  a  tall  conical 
chimney  which  stands  over  the  entire  chamber  and  com- 
municates with  it  by  a  number  of  small  openings.  In  a 
more  refined  furnace,  the  chamber  itself  is  double,  and  the 
flame,  after  playing  around  the  crucible  in  the  inside  of  the 
chamber,  is  made  to  pass  through  the  space  between  the 
outer  and  inner  chamber  before  passing  to  the  chimney  or 
cone.  We  need  not  give  any  greater  attention  to  these 
primitive  furnaces,  since  they  are  practically  obsolete  at  the 
present  time.  In  modern  furnaces  the  process  of  com- 
bustion is  carried  on  in  two  distinct  stages  ;  the  first  stage 
takes  place  in  a  subsidiary  appliance  known  as  a  "  gas  pro- 
ducer," where  part  of  the  heat  which  the  fuel  is  capable  of 
generating  is  utilised  for  the  production  of  a  combustible 


CRUCIBLES  AND  FURNACES  FOR  FUSION  OF  GLASS.    63 

gas  ;  this  gas  passes  into  the  furnace  proper,  either  direct, 
while  it  is  still  hot  from  the  producer,  or  after  being  con- 
veyed some  distance,  when  it  is  again  heated  up  by  the 
waste  heat  of  the  furnace.  In  either  case  the  gas  is  hot 
when  it  enters  the  furnace  proper,  and  there  it  meets  a 
current  of  air,  also  heated  by  the  aid  of  the  waste  heat  of 
the  furnace.  Hot  gas  and  hot  air  burn  rapidly  and  com- 
pletely, and  if  properly  proportioned  yield  exceedingly  high 
temperatures.  Seeing  that  in  this  process  a  part  of  the 
heat  of  combustion  yielded  by  the  fuel  is  generated  in  a 
subsidiary .  appliance  and  is  thus  lost  to  the  furnace,  it 
appears  at  first  sight  somewhat  surprising  that  this  system 
of  firing  is  very  considerably  more  efficient  than  the  old 
"  direct "  system  where  the  whole  of  the  fuel  is  burnt  in 
the  furnace  itself.  But  the  advantage  arises  from  the  fact 
that  in  the  newer  system  the  fuel  is  handled  in  the  gaseous 
form.  This  has  the  advantage,  first  and  most  important, 
that  the  heat  escaping  from  the  furnace  in  the  hot  pro- 
ducts of  combustion  (chimney  gases)  can  be  transferred  to 
the  incoming  unburnt  gas  and  air  and  can  thus  be  returned 
to  the  furnace.  The  manner  in  which  this  is  accomplished 
will  be  considered  below,  but  it  may  be  noted  here  that  in 
some  furnaces  the  escaping  products  of  combustion  are  so 
thoroughly  cooled  that  they  are  unable  to  produce  an 
effective  draught  in  the  chimney  of  the  furnace.  Another 
advantage  of  the  use  of  gaseous  fuel  is  the  fact  that  com- 
plete combustion  can  be  obtained  without  the  use  of  so 
great  excess  of  air,  such  as  is  required  when  solid  fuels  are 
to  be  burnt  completely.  For  this  reason  much  higher 
temperatures  can  be  readily  obtained  with  gaseous  fuel, 
while  the  pre-heating  of  both  gas  and  air  also  facilitates  the 


64  GLASS  MANUFACTURE. 

attainment  of  high  temperatures  ;  further,  the  great  facility 
with  which  the  flow  of  either  gas  or  air  can  be  regulated  by 
means  of  suitable  valves,  makes  it  possible  to  secure  much 
greater  regularity  in  the  working  of  the  furnaces.  Finally, 
in  modern  gas-producers,  the  amount  of  sensible  heat 
generated  and  therefore  lost  to  the  furnace,  is  kept  very 
low,  the  greater  part  of  the  heat  set  free  by  the  partial 
combustion  of  coal  in  the  producer  being  absorbed  by  the 
decomposition  of  a  corresponding  quantity  of  steam  into 
hydrogen  and  carbonic  oxide  gas.  The  gas  as  it  leaves  one 
of  these  producers  is  not  very  hot,  and  the  percentage  of 
heat  lost  in  this  way  is  therefore  much  smaller  than  in  the 
older  forms  of  gas-producer. 

It  is  again  impossible,  within  the  limits  of  this  chapter, 
to  enter  into  the  details  of  construction  and  working  of 
gas-producers.  We  must  content  ourselves  with  saying  that 
most  modern  producers  are  of  the  form  of  a  tower  in  which 
a  thick  bed  of  fuel  is  partially  burnt  and  partly  gasified 
under  the  action  of  a  blast  of  air  mixed  with  steam.  The 
chemical  actions  that  take  place  are  complicated,  but  the 
final  result  is  the  production  of  a  gas  containing  from  2  to 
8  or  10  per  cent,  of  carbonic  acid,  10  to  20  per  cent,  of 
hydrogen,  8  to  25  per  cent,  of  carbonic  oxide  (CO),  1  to  3 
per  cent,  methane  (CH4),  and  45  to  60  per  cent,  of  nitro- 
gen, with  varying  quantities  of  moisture,  tarry  matter,  and 
ammonia.  In  good  producer  gas,  the  combustible  con- 
stituents (hydrogen,  carbonic  oxide  and  methane)  should 
total  from  30  to  48  per  cent,  of  the  whole  by  volume,  but 
the  exact  composition  to  be  expected  depends  very  much  on 
the  type  of  producer  and  the  class  of  fuel  used.  Some  pro- 
ducers are  capable  of  dealing  with  exceedingly  low-grade 


CKUCIBLES  AND  FUENACES  FOE  FUSION  OF  GLASS.    65 

fuels,  and  the  gas  which  they  yield  can  still  be  utilised  for 
obtaining  the  highest  temperatures — a  proceeding  that 
would  have  been  impossible  if  it  had  been  attempted  to 
burn  these  fuels  directly  in  the  furnace. 

The  gas  on  leaving  the  producer  passes  along  fire-brick 


REGENERATOR 


REGENERATOR 


FIG.  3.  —  Diagram  of  the  arrangements  of  a  regenerative  furnace. 


flues  or  passages  to  the  furnace  proper  ;  the  path  which  it 
is  now  caused  to  take  varies  somewhat  according  to  the 
arrangement  of  the  furnace  in  question.  Modern  gas-fired 
furnaces  usually  belong  to  one  of  two  distinct  types  accord- 
ing to  the  manner  in  which  the  heat  of  the  escaping  pro- 
ducts of  combustion  is  utilised  for  heating  the  incoming 
gas  and  air  ;  these  two  types  are  known  as  the  "  regenerative  " 
G.M.  F 


66  GLASS   MANUFACTURE. 

and  the  "recuperative"  respectively.  In  regenerative 
furnaces  the  hot  products  of  combustion,  after  leaving  the 
furnace  chamber  proper,  and  before  reaching  the  chimney, 
pass  through  chambers  which  are  loosely  stacked  with  fire- 
bricks; these  chambers  absorb  the  heat  of  the  escaping 
gases,  and  thus  rapidly  become  hot.  As  soon  as  a  sufficiently 
high  temperature  is  attained  in  these  chambers  or  "  re- 
generators," the  path  of  the  gas-currents  is  altered  ;  the 
escaping  products  of  combustion  are  made  to  pass  through, 
and  thus  to  heat  a  second  set  of  regenerating  chambers, 
while  the  incoming  gas  and  air  are  drawn  through  the 
heated  regenerator  chambers  before  entering  the  furnace 
chamber  proper.  The  incoming  gas  and  air  are  thus  heated, 
absorbing  in  turn  the  heat  stored  in  the  brickwork  of  the 
regenerators.  It  is  evident  that  two  sets  of  such  regenera- 
tors are  sufficient,  the  one  set  undergoing  the  heating 
process  at  the  hands  of  the  escaping  products  of  combustion, 
while  the  other  set  is  giving  up  its  heat  to  the  incoming  gas 
and  air;  when  this  process  has  gone  far  enough,  it  is  only 
necessary  to  interchange  the  two  sets  of  chambers,  by  the 
operation  of  suitable  valves,  and  this  series  of  alternations 
may  be  continued  indefinitely.  The  arrangement  is  shown 
diagrammatically  in  Fig.  3. 

In  recuperative  furnaces  the  same  principle  is  utilised  in 
a  somewhat  different  manner  ;  the  outgoing  products  of 
combustion  pass  through  tubular  channels  formed  in  fire- 
clay blocks,  while  the  ingoing  gas  and  air  pass  around  the 
outside  of  these  same  blocks;  the  heat  of  the  outgoing 
gases  is  thus  transferred  to  the  incoming  gases  by  the 
process  of  conduction  through  the  fire-clay  walls  of  the 
recuperator  tubes.  The  relative  merits  of  the  two  systems 


CEUCIBLES  AND  FUENACES  FOE  FUSION  OF  GLASS.     67 

have  been  hotly  contested ;  the  regenerative  system  has  the 
advantage  of  avoiding  all  reliance  on  the  heat  conductivity 
of  fire-clay,  while  it  also  avoids  the  somewhat  complicated 
special  tubular  blocks  required  for  the  other  system ;  on 
the  other  hand,  the  recuperative  system  avoids  the  necessity 
for  all  "  reversing "  valves  and  their  regular  periodical 
working,  while  it  also  occupies  somewhat  less  space. 
Temperatures  sufficiently  high  for  all  glass-melting  pur- 
poses can  be  attained  by  both  means. 

In  both  systems  of  furnace,  heated  gas  and  heated  air 
are  admitted  to  the  furnace  by  separate  fire-brick  flues  or 
passages,  air  and  gas  being  allowed  to  mix  just  before  they 
enter  the  furnace  chamber  proper.  The  economy  and 
efficiency  of  the  furnace  depend  to  a  very  great  extent  upon 
the  manner  in  which  this  mixing  is  accomplished.  Eapid 
and  complete  mixing  of  air  and  gas  results  in  an  intensely 
hot,  but  short  and  local  flame,  while  slower  mixing  tends  to 
lengthen  the  flame  and  spread  the  heat  through  the  entire 
furnace  chamber  ;  on  the  other  hand  if  the  mixing  of  gas 
and  air  is  too  slow,  combustion  may  not  have  been  com- 
pleted in  the  short  time  occupied  by  the  gases  in  passing 
through  the  furnace,  and  combustion  may  either  continue 
in  the  outflow  flues  and  regenerators,  or  it  may  be  prevented 
by  the  narrowness  of  these  passages,  and  unburnt  gases  may 
pass  to  the  chimney.  When  the  openings  or  "ports  "  are 
properly  proportioned,  and  the  draught  of  the  chimney  is 
properly  regulated,  combustion  should  be  just  complete  as 
the  gases  leave  the  furnace  chamber,  and  under  these  cir- 
cumstances small  tongues  of  keen  flame  will  escape  from 
every  opening  in  the  furnace ;  large  smoky  flames  issuing 
from  a  gas-fired  furnace  indicate  incomplete  combustion. 

F  2 


68 


GLASS  MANUFACTUEE. 


As  has  already  been  indicated,  glass  is  melted  either  in 
pots  or  crucibles  of  various  shapes  and  sizes,  or  in  open 
tank  furnaces.  The  general  arrangement  of  a  pot  furnace 
working  with  closed  or  "  covered  "  crucibles  is  shown  in 
Fig.  4.  In  this  particular  furnace,  the  "  ports  "  or  aper- 
tures by  which  the  gas  and  air  enter  the  furnace  chamber, 
are  placed  in  the  floor  of  the  chamber,  but  these  apertures 


FIG.  4. — Sectional  diagram  of  a  regenerative  pot  furnace  working  with     , 

covered  pots. 

are  often  placed  in  the  side  or  end  walls,  or  even  in  a  central 
column,  the  object  being  in  all  cases  to  heat  all  the  pots  as 
uniformly  as  possible  and  to  avoid  any  intense  local  heating, 
which  would  merely  endanger  the  particular  crucible 
exposed  to  it,  without  greatly  aiding  the  real  work  of  the 
furnace.  In  pot  furnaces,  however,  in  which  the  more 
refractory  kinds  of  glass  are  to  be  melted,  it  is  generally 
considered  desirable  that  the  flame  should  be  made  to  play 


CRUCIBLES  AND  FUENACES  FOR  FUSION  OF  GLASS.    69 

about  the  pots  in  such  a  way  as  to  heat  the  lower  parts  of 
the  pots  most  strongly.  In  connection  with  the  question  of 
the  uniformity  of  heat  distribution  in  a  gas-fired  furnace  it 
must  further  be  borne  in  mind  that  in  the  case  of  regenera- 
tive furnaces  the  direction  of  the  flame  is  reversed  every 
time  the  valves  are  thrown  over,  and  in  practice  this  is  done 
about  once  every  half-hour ;  this  proceeding,  of  course, 
tends  very  much  to  equalise  the  temperature  of  the  two 
sides  of  the  furnace.  In  recuperative  furnaces,  on  the  other 
hand,  the  direction  of  the  flame  is  not  changed,  and  for 


FIG.  5. — Diagram  of  a  furnace  with  "  horse-shoe  "  flame. 

that  reason  a  flame  returning  upon  itself,  usually  called  a 
horse-shoe  flame,  is  often  employed;  this  is  obtained  by 
placing  the  entry  and  exit  ports  side  by  side  at  one  end  of 
the  furnace  ;  the  impetus  of  the  flame  gases  and  their  rapid 
expansion  during  combustion  carry  the  flame  out  across 
the  furnace,  while  the  chimney  draught  ultimately  sucks  it 
back  to  the  exit  ports,  the  shape  of  the  flame  being  shown 
in  Fig.  5. 

In  general  arrangement,  a  tank  furnace  for  glass  melting 
resembles  an  open-hearth  steel  furnace.  The  tank  or  basin, 
as  already  indicated,  is  built  up  of  a  number  of  large  fire-clay 


70 


GLASS  MANUFACTUKE. 


blocks,  forming  a  bath  varying  in  depth  from  20  in.  to 
42  in.  according  to  the  design  of  the  furnace  and  the  kind 
of  glass  to  be  melted  in  it.  The  ports  for  entry  of  gas  and 
air  and  for  exit  of  the  products  of  combustion  are  in  most 
modern  furnaces  placed  in  the  side  walls  of  the  furnace 
just  above  the  level  of  the  glass,  the  whole  being  covered  by 
a  vault  built  of  silica  brick.  Figs.  6  and  7  show  the  general 
arrangement  of  a  simple  form  of  tank-furnace  such  as  that 
used  in  the  manufacture  of  rolled  plate  glass.  The  furnace 


FIG<  6. — Longitudinal  sectional  diagram  of  tank  furnace. 

indicated  in  the  diagram  is  intended  for  regenerative  work- 
ing with  alternating  directions  of  flame ;  in  recuperative 
furnaces  the  horse-shoe  flame  is  always  used  in  tanks, 
while  this  arrangement  of  ports  is  sometimes  adopted  for 
regenerative  tanks  also,  particularly  in  the  manufacture  of 
bottles.  For  the  production  of  sheet  glass,  tank  furnaces 
are  generally  subdivided  into  two  compartments  and  are 
also  provided  with  various  constrictions  intended  to  arrest 
impurities  and  to  allow  only  clear  glass  to  pass,  but  as 
regards  the  arrangement  of  flues  and  ports  there  is  a  very 
general  similarity  between  various  furnaces  of  this  type. 


CRUCIBLES  AND  FUENACES  FOE  FUSION  OF  GLASS.     71 

It  is  beyond  the  scope  of  this  book  to  discuss  the  relative 
merits  of  tank  and  pot  melting  furnaces ;  wherever  the 
former  can  be  made  to  produce  glass  of  adequate  quality  for 
the  purpose  desired,  the  great  economy  of  the  tank  furnace 
inevitably  carries  all  before  it,  so  that  bottle  glass,  for 
example,  is  now  made  exclusively  in  tanks,  and  the  same 


FIG.    7. — Transverse    sectional    diagram  of    tank   furnace,    showing 
regenerators  and  gas  and  air  passages. 

applies  also  to  rolled  plate  of  the  ordinary  kind,  and  to  the 
great  majority  of  sheet  glass.  On  the  other  hand,  where 
special  qualities  of  glass  are  required  in  relatively  small 
quantities,  or  where  the  requirements  as  to  quality  are  very 
extreme,  the  pot  furnace  remains  indispensable.  Optical 
glass  and  coloured  glasses  are  examples  of  this  kind, 
although  some  tinted  glasses  are  used  in  sufficient  quan- 
tity to  justify  the  use  of  small  tank  furnaces  for  their 


72  GLASS   MANUFACTURE. 

production.  The  causes  of  the  greater  economy  of  the  tank 
furnace  are  numerous,  and  complicated  by  the  detailed 
requirements  of  each  particular  manufacture,  but  the  most 
important  factors  in  the  question  may  be  summed  up  thus  :— 

(1)  The  tank  furnace  utilises  the  heat  of  the  flame  more 
efficiently,  as  the  glass  is  exposed  to  the  heat  in  a  basin 
whose  surface  covers  the  entire  area  of  the  furnace,  while 
in  a  pot  furnace  there  is  much  vacant,  unused  space. 

(2)  The  tank  furnace  permits  of  continuous  working,  the 
raw  materials  being  introduced  at  one  end  while  the  glass 
is  being  withdrawn   and  worked  at  the  other  end.     There 
are  thus  no  idle  periods,  and  each  part   of   the  furnace 
remains  at  or  near  the  same  temperature  during  the  whole 
time  that  a  furnace  is  alight.     For  a  given  size  of  plant, 
therefore,  a  tank  furnace  yields  a  much  larger  output,  with 
a  relatively  smaller  fuel  consumption. 

(3)  The  tank  furnace  obviates  the  need  for  pots  or  crucibles, 
which  are  not  only  costly  and  troublesome  to  produce,  but 
are  liable  to  premature  failure  and  require  periodical  re- 
newal, which  involves  a  serious  loss  of  time  for  the  furnace. 

(4)  Finally,  the  molten  glass  in  a  tank  furnace  can  be 
always  maintained  at  or  near  one  constant  level  and  is, 
therefore,  always  convenient  for  withdrawal  by  means  of 
the  gatherer's  pipe  or  the  ladle. 

In  pot  furnaces,  on  the  other  hand,  the  composition  of 
the  glass  can  be  more  accurately  regulated,  and  the  molten 
glass  itself  can  be  more  effectively  protected  from  contami- 
nation either  by  matter  dropping  into  it  or  by  the  action  of 
the  furnace  gases,  while  in  pots  it  is  also  possible  to  effec- 
tually melt  together  materials  which,  in  the  open  basin  of 
a  tank,  could  not  be  kept  together  long  enough  to  combine. 


CHAPTEK    V. 

THE    PROCESS    OF    FUSION. 

IT  has  already  been  indicated  that,  for  glass-making  pur- 
poses, the  raw  materials  are  required  in  a  state  of  reasonably 
tine  division.  The  exact  degree  of  fineness  required  depends 
very  much  upon  the  nature  of  the  ingredient  in  question, 
the  general  rule  being  that  the  more  refractory  and 
chemically  resistant  materials  require  to  be  most  finely 
ground,  while  substances  which  melt  and  react  readily, 
such  as  soda  ash  and  salt-cake,  do  not  require  very  fine 
grinding. 

Assuming  that  the  materials  are  available  in  a  suitable 
state  of  fineness,  the  first  step  in  the  process  of  glass  melt- 
ing consists  in  securing  their  admixture  in  the  proper 
proportions.  This  may  be  done  by  hand  entirely,  by  hand 
aided  by  some  machinery,  or  entirely  automatically.  The 
process  of  hand  mixing  is  only  available  for  relatively  small 
quantities  of  material  and  requires  very  careful  supervision 
if  inadequate  mixing  is  to  be  avoided.  In  most  cases  the 
actual  weighing  out  is  done  by  hand,  while  the  mixing  is 
done  by  machinery.  In  this  process  the  separate  ingre- 
dients are  weighed  out  from  barrows  or  skips  and  are 
tipped  into  a  large  hopper  whence  each  batch,  as  soon  as  it 
is  completed,  passes  into  the  mixing  chamber  of  the  mixing 


74  GLASS   MANUFACTUEE. 

machine.  This  may  consist  of  nothing  more  than  a  cylin- 
drical chamber  in  which  steel  arms  revolve  and  stir  up  the 
contents,  but  more  modern  appliances  take  the  form  of 
rotating  barrels  or  cylinders,  set  up  on  an  inclined  axis  and 
provided  with  suitable  shelves  and  baffles  ;  in  these  the 
materials  are  very  thoroughly  shaken  over  and  mixed. 
Where  hand  mixing  is  adopted,  the  various  ingredients  of 
each  batch  are  thrown  into  a  large  bin  and  are  there  turned 
over  several  times  with  shovels,  the  entire  material  being 
ultimately  sieved  through  a  wire  sieve  of  suitable  mesh. 
In  all  cases  the  resulting  mixture  should  be  perfectly 
uniform  in  colour  and  texture,  and  analyses  of  different 
samples  should  show  only  small  variations.  With  the 
mixture  thus  prepared  the  "  cullet  "  or  broken  glass  which  is 
to  be  re-melted  is  now  incorporated  ;  ideally  this  should 
also  be  uniformly  distributed,  but  this  is  rarely  attempted 
in  practice  on  the  large  scale. 

The  next  step  in  the  process  is  the  introduction  of  the 
mixture  into  the  furnace.  In  the  case  of  tank  furnaces 
this  is  a  simple  matter,  since  in  these  the  temperature  is 
kept  as  nearly  constant  as  possible,  and  raw  materials  may, 
therefore,  be  introduced  at  almost  any  time,  the  amount 
introduced  being  so  regulated  as  to  keep  the  level  of  the 
molten  glass  or  "metal"  as  nearly  constant  as  possible. 
The  actual  introduction  is  managed  by  means  of  a  large 
opening  or  door  at  what  is  known  as  the  "  melting  end  "  of 
the  furnace.  Normally  this  opening  is  covered  by  a  large 
firebrick  block  suspended  by  a  chain  running  over  pulleys 
and  counterbalanced  by  a  counterpoise  weight.  When 
charging  is  to  begin,  this  block  is  raised  and  the  opening  is 
uncovered.  The  raw  materials  are  then  introduced  either 


THE  PEOCESS   OF  FUSION.  75 

by  hand,  by  the  aid  of  long-handled  shovels,  or  they  are 
first  filled  into  a  long  scoop  moved  by  mechanical  means 
forward  into  the  furnace,  where  it  is  given  a  half-turn, 
which  empties  the  contents  out,  and  is  then  rapidly 
withdrawn. 

This  charging  process  may  be  repeated  every  half-hour, 
or  larger  quantities  may  be  introduced  once  every  four 
hours,  according  to  the  practice  that  may  be  adopted  at 
any  particular  furnace. 

In  the  case  of  pot  furnaces  the  charging  process  is  not  so 
simple.  Here  the  first  charge  of  raw  materials  has  to  be 
introduced  into  a  pot  which  has  been  almost  entirely 
emptied  during  the  working-out  process,  and  the  tempera- 
ture of  the  furnace  has  also  fallen  very  considerably  during 
this  time.  Before  new  material  is  introduced,  the  heat  of 
the  furnace  must  first  be  adequately  restored.  If  this  is 
not  done,  the  fusion  of  the  glass  takes  an  abnormal  course 
and  very  imperfect  results  arise.  Further,  the  quantity  of 
material  introduced  at  one  time  must  be  carefully  adjusted 
to  the  capacity  of  the  pot.  During  the  earlier  stages  of 
fusion  most  glass  mixtures  form  large  masses  of  foam,  and 
if  the  crucible  has  been  too  heavily  charged  this  foam  over- 
flows, with  the  result  that  valuable  material  is  lost  and  the 
floor  and  passages  of  the  furnace  are  clogged  with  glass. 
A  certain  amount  of  overflow,  as  well  as  leakage  from 
defective  crucibles,  is,  however,  unavoidable,  and  for  this 
purpose  every  pot  furnace  is  provided  with  a  chamber  so 
placed  that  the  glass  will  flow  into  it  and  so  be  prevented 
from  finding  its  way  into  the  regenerators  or  other  parts 
where  its  presence  would  hinder  the  working  of  the 
furnace.  These  receptacles  or  ''pockets  "  must,  however,  be 


76  GLASS  MANUFACTUEE. 

periodically  cleared  of  their  contents  from  outside,  and  this 
constitutes  one  of  the  most  irksome  operations  connected 
with  glass  manufacture.  Owing  to  the  occurrence  of 
foaming  and  to  the  fact  that  the  raw  materials  occupy 
much  more  space  than  the  glass  formed  from  them,  it  is 
necessary  to  fill  the  pot  with  fresh  batches  of  raw  materials 
several  times,  the  quantity  which  can  be  introduced 
decreasing  each  time.  The  number  of  times  that  this 
must  be  done  depends  upon  the  particular  circumstances, 
but  from  four  to  eight  "  fillings  "  are  commonly  used  for 
various  kinds  of  glass  and  size  of  pot.  The  precise  stage 
at  which  a  fresh  batch  of  raw  materials  should  be  intro- 
duced is  another  matter  requiring  careful  attention.  For 
some  purposes  it  is  necessary  to  wait  until  the  previous 
batch  is  completely  melted,  while  in  other  cases  raw 
material  may  be  added  whilst  some  of  the  previous  batch 
is  still  floating  on  the  surface  of  the  glass  in  the  pot. 

We  have/  now  to  consider  the  chemical  reactions  which 
take  place  in  the  mixture  of  raw  materials  that  are  intro- 
duced into  the  hot  furnace.  The  exact  course  of  these 
reactions  is  not  known  in  very  great  detail,  as  this  could 
only  be  ascertained  by  an  elaborate  research  on  the  nature 
of  the  intermediate  products  that  result  under  various 
circumstances.  A  research  of  this  kind  would  throw  much 
light  on  the  whole  of  the  melting  processes  but  is  in  itself 
so  difficult  that  it  has  not  yet  been  carried  out  at  all  fully. 
We  can  therefore  only  give  an  account  of  the  chemical 
changes  from  our  knowledge  of  the  end-results  and  of  a 
few  intermediate  products  that  are  known.  To  take  the 
simplest  case,  we  may  consider  a  mixture  consisting  of 
sand,  carbonate  of  lime  and  carbonate  of  soda  mixed  in 


THE  PEOOESS  OF  FUSION.  77 

suitable  proportions.  In  such  a  case  we  know  that  the  mere 
action  of  heat  alone  will  produce  two  changes — the  carbon- 
ate of  soda  will  melt  and  the  carbonate  of  lime  will  lose  its 
carbonic  acid  and  be  "  burnt  "  or  converted  into  caustic 
lime.  The  first  stage  of  the  fusion  process  thus  probably 
results  in  a  mass  consisting  of  sand  grains  and  grains  of 
carbonate  of  lime  undergoing  decomposition,  all  cemented 
together  by  molten  carbonate  of  soda.  This  mass  will  be 
full  of  bubbles,  some  derived  from  the  air  enclosed  between 
the  grains  of  the  original  mixture  and  thus  trapped  by  the 
melting  mass,  and  others  formed  by  the  carbonic  acid 
which  is  being  driven  off  in  the  form  of  gas  by  the  decom- 
position of  the  carbonate  of  lime.  At  the  temperature  of 
the  furnace,  however,  silica  has  the  properties  of  a  strong 
acid,  and  not  only  attacks  the  carbonate  of  lime  much  in 
the  same  manner  as,  for  instance,  hydrochloric  acid  would 
do  in  the  cold,  but  the  silica  also  attacks  the  carbonate  of 
soda,  which  heat  alone  can  scarcely  decompose.  The  exact 
order  in  which  these  reactions  take  place  will  depend  upon 
the  temperature  of  the  furnace  and  the  degree  of  mixing 
attained  in  the  preparation  of  the  raw  materials.  Although 
in  the  long  run  the  final  result  will  probably  be  the  same 
as  regards  purely  chemical  constitution,  much  of  the 
technical  success  of  the  process  must  depend  upon  the 
exact  sequence  of  the  changes  involved,  as  this  must  govern 
the  number  and  size  of  the  bubbles  that  are  formed  in  the 
glass  and  the  fluidity  of  the  mass  from  which  these  bubbles 
have  to  free  themselves.  In  the  present  state  of  our 
knowledge,  however,  we  can  only  say  that  the  final  result 
is  the  complete  expulsion  of  all  carbonic  acid  from  the 
compounds  present  (although  it  may  remain  entangled  in 


7N  LANN   MANHi   \<  TIIIM; 

Mm    {diUlil    ill    Mir    h.im    .,1     hnU.lr    >     u,.|     il,,      lunnnhun    .,1 

HJIicillcH   of    linMl    Illlli    lllpl    ilDiln.   Wllirli    l<   Ili.Hli    111    Mir    li  1 1  I.  ,1 1«  •<  | 

f/lll.HH    itt    li    Htllto    plU'l/ly    of    inulmil    rlirniirn.l     coinlminl  ion, 

partly  of  iiiiiliml  Holution, 

'I'll''  •   d<      «   I  IpMoli    n|'    MM      |.|.M  cMHOf  fllHJOll  jllHl,  |MVnil  M.p|»llr .:. 

wifch  illght  modiflottloni,  i.->  Mm  mrihn,"  .,r  ordinary  iimi, 

,".!.':      mi  \tllWH    IIH    W()ll     HH    i()     liniO    glllHHOH,     \VlMl     MIC     our 

liliciii  ion    I|MI.    j|l(     .  .11  |,,,n.ii.     of    IIIIH     ol    Mir    I i  Hodn 

I'M  ropliin»<l   l»(y  rod  Icn.l,   mid    Mir   ,-n     ovolvod    )*v    Mm 

I'        |||,  M     ,|       |(.;|(|          |  ;;        I,        ^    ,.,     I,          I,,          |,  |;ir(,       ,,|         |    |l(. 

(Mtl'l)(>lll«-|M-ld   OVolVod  IV«  'Ill  Mid  (|rroill|)(»!ilMuil  ol'  Mir  c.n  |i«,|i,ilr 

of  liniUi     In  MIC  ('iiHiM)l' IxiMi  liinniuul  flint gliiHMOH,  howovor, 

I'M  Inn  ..l.lirr  MIlliHtlUKMiM  !)<•!, idcii  Miniir  nini  I  i.  .nr.  I  .iir  II..U;I||N 
inliiMliicril  in  Mlliull  (JlllUliitioH,  Altlioiii'h  l.lioiut  nnli.iln.iirr:: 

•I"      n.. I.       \ny      in.il.  i  i  ill  ,      nJlrrl,     Mir     rii.l    | hid        ,.l      Mir 

cJlOlllirilJ     roiirl.iniiM,     Mirv\ci\     in.ili  i  Lilly    ;i.lVrrl,     Mir     illtor- 

liiodiuto  HliiijMiM,  iMid  Minn  imrvo  tlm   |>m|>oMn   for  wliii'li    Mi.  \ 

."ir  mil  oilucrd  |»y  JillrrMii;'  Mir  OHll'MO  (>f  Mir  rl  ir  in  irn.l 
dllltlgOH  Mi  n.  I'M  \  oiiniJtlr  in  inn-  i  'I'lir  MI  |i;;|,)Mi('()H  lltilinll\ 
(Mll|»|n\  i  .1  I.. i  Mil,.  |>ii!|>...  <•  ;i.ir  ,Ml::rlilc  nihl  nih.ilr  <>|  rllliri 
Modll  Ol'  poillhll.  Tlir  IIIMIIIHM'  Hi  \\ln.li  MM-  ;u::rllir  ;i-|  i 
\<i\  "|».  «  in  ,•  :in,|  r.iiMli>r  )>r  1 1  I,  :r  1 1 ; ;;  :n  I  It:  drln.il  llrlT.  Mir 
rllirf  Indol'M  III  ll..  .iclnili  ;ilr,  ln>\\r\ri.  ii...  \<>l.ililii\  :iinl  si,: 
|M)\Vnr  of  riMiri  n  1 1; ,» »i  I  >|  n;;  o\\;;rii  MI  JDI.!'!.!)!;',  \\illi  it  n.rcoi'd  • 
illg  to  rirrmn;.! ;i in  <•;;  Tin  .iriimi  ol  Mir  niii.iir..  i  .  rliiolly 

dltpnildoilt  upon   II \-.  n    \\lucli    Mi.  \    \irl.l   on  dlUMXIIpOMl'- 

l-i"M  I'V  lirnt.  Till.';  o\\  "rn  n:  ill  ;:oinr  r:i;r;;  ;,loird  ll|)  |iy 
otluu'  iii,",i  (  ili.nl  .  of  Mir  mi. I. urn  mid  only  .".ivnn  off  n.l>  it 
Illlirli  Inlnr  !i|,M.,",n,  \\lirli  M  ir  r  \  <  >l  ll  I  h  >l  I  ol'  MM  ;-n:;  ;i;.;.|:.|;;  in 
Mir  rnmovnl  ol  Mir  In.:. I  .in  ill  l.nl.l.lr  .  ol  inn  I  .111  or  c:i.rl»on  ir 
•"  ''I  ",''  '  -Mil  Irll  in  (ho  glllHH.  Tllr  oXhll.MIl;-  .iclioll  of  Mir 


Till'!    IMUH'I'KS  OK    KIIHION.  79 

mh;i.f<     .      h'.ucvcl1,     HnrVJW     '•liully      l"l      tin-     <l<     I  i  n« -I  KMI     of 

or;';mi«-  mill  ice  mid  (.In-  lull  oxidat  ion   o!' any    iron    pniiiont  ; 

hol.h  |K  (>(•(.,;(•:•  which  lili'l  l<(  lln|»r<r.<  flu  •  •n\'i\\\  i,\  t})() 
JJjIllHH,  \v)ll  I'  III  I  IK  <  .'  <  '.I  Mini  ^|||,HH<V  III'  |>i<  <  M'  <  "I  I  IK  < 

0  i'li  in;-  additionn  JH  imcwHHary  to  avoid  all  rink  ol  reduction 
Of  lead  in'1  l  In  would  ivmilt  in  UK  ci,ni|ilH.<)  hlack<minjj( 
"I  I  ho  ^IllMH. 

A    iiiii'-h    HIOK      ••(,ni|)lic;il«  -I       <l     '.I     niUCiioriH    ocelli1    wluMI 

id*   iilkitli  "I  n  Hoda-limo  ^!«IHH  in  intm.lur,  <|   <'ith<.r  |»;nil;, 

or  wholly   in   tin    I'orni  ol  r,iil|ih;il,c   of   ;,od;i    i-.-.-.t  II,  ruKi-j.       \V<: 

;i.lr«'M.dy    pom  ( ('I    oil  I,    lli;il     MM      Illlllidcd    .KIK.N     -,l     I  K.I  I 

.OK!    ',1     iliofl    i      IK. I      ulli.-i.  MI    tu    hrin^  nhonl.    Uio    rapid 

d«  coni|;o;  it  ion  '.I  •  iilplcid  '.I  o<ln  v,  In-  li  !H  M  <|IIIM  'I  i"i 
HUCCOMHful  f^lliHH  HlliHIli'iirl.uri  .md  lh;il  UK  i  n  I  <  i  M  1 1 1  K/H  ol' 
reducing  a^^nl/H  i  i«'|inn:d.  KOI  llu  purpOHt!  ;i  '••ttiiin 

lunonni  «<l  c.ul.'Mi  m  (In  Conn  of  <!ol(o,  (;ha»'(5oal  or  anihnu;ito 
•"•.'I  i  [ntfOdttC0(]  into  all  MaliriiKc  nio.l.inv;.;,  hnl  UK: 

IC'lil'-m,"  ;-.,  <  ',1  Hie  |iini;icc  ;I||MO--.|I|II  M  lljHO  play  ail 
impoihml  |/;irl  in  lh«  r«-;M-!.ioiiH  lh;il,  l;il.«  pliMT.  llcr<: 

iif'Hin  ii  i    nol  )/'.    ihl<    lo  i  Inn;-  I, MI    mi    inwmiph  t« 

1    '-'        ;   •    <     l.ii  «         |.l;i»-<  T(K      i;,l  M,I:.,|<      <,|     ||M-     \vhol«' 

prOCGHH    litiH,    no    doiihl.,    in     lh<      l;i<-|     ||,.,|     :iilpliih     '.I     Hoda 

(Na.xHO;,)  i«  nnfh  inon  readily  d<'''onij;ori<'(|  hy  Un;  nd.ion  of 
""<<  iilicn  Lhiiti  i!.«  Hulphato  (\a,S<).,)  itnolf,  HO  Uiai  UK 
'  ;-'nii;.l  ariion  '.I  ih«  mdurJug  agwitu  (jonniKl/H  in  rohhin^ 
1-lnMiiilpfiali  -.I  parl  -.1  it  ,  oxy^jn,  1/hiJH  njdiKjiug  it  to  lh«- 
condition  «,l  ;-.ulphilc  :md  n-iid«-rin/;  il  ;if«-«;nMJI>li)  to  tho 
:'IL"'1  "'  ilicic  iicidi  hni  il  w<  uUiinipl  to  <;xpreHH  HIK-I-  i 
"  ;"-ii"n  in  i  IK  n  M. J  tnitnix  i  hy  ;i  »-h«  niicji.1  <  fjiiution  from 

v  lii'-li  I  h«  «|n:mf  il.y  of  f-;n  hon  i<  'i  HIM  <\  lo  i  ||<  d  I  IK  n  die-lion 
i"  (|li(  h""  '  •  |(  "lilt(  'I  ••'•'  find  fhiif  i  IK  .iniounl  "I 


80  GLASS  MANUFACTURE. 

carbon  required  in  practice  is  very  considerably  less  than 
that  given  by  this  theory ;  it  follows  therefore  that  either 
this  very  large  amount  of  reducing  action  must  be  ascribed 
to  the  furnace  gases,  or  that  the  actual  reactions  are  not 
strictly  of  the  kind  we  have  described.  Both  explanations 
are  probably  partly  correct,  and  in  practice  the  amount  of 
carbon  to  be  used  in  a  given  mixture  and  furnace  can  only 
be  found  by  actual  trial,  in  which  the  manufacturer  is,  of 
course,  guided  by  the  results  obtained  with  other  furnaces 
of  a  similar  type.  The  end-product  of  the  reactions  is 
again  a  mixture  of  silicates,  but  a  certain  amount  of  unde- 
composed  sulphate  is  always  found  in  such  glasses,  while 
gaseous  oxides  of  sulphur  escape  from  these  furnaces  in 
considerable  quantity.  Under  exceptional  circumstances 
the  glass  may  even  contain  sulphides  of  soda  or  of  lime, 
and  sometimes  even  suspended  carbon,  but  these  are 
abnormal  constituents  and  result  in  the  serious  discolouration 
of  the  glass. 

It  is  obvious  that  to  a  mixture  containing  carbon  as  a 
reducing  agent  such  oxidising  materials  as  nitrates  cannot 
be  added,  but  small  quantities  of  arsenic  and  of  manganese 
dioxide  are  added  because  their  other  properties  are 
sufficiently  valuable  to  outweigh  their  disadvantages  as 
oxidising  agents. 

Having  now  briefly  considered  the  process  of  fusion 
proper,  we  pass  to  the  second  stage  in  the  melting  of 
glass.  In  a  properly  conducted  glass-furnace,  when  the 
last  trace  of  undecomposed  raw  materials  has  disappeared, 
we  find  the  glass  as  a  transparent  mass  throughout  which 
gas  bubbles  are  thickly  disseminated.  For  the  majority  of 
purposes  it  is  necessary  to  free  the  glass  as  perfectly  as 


THE  PEOCESS  OF  FUSION.  81 

possible  from  these  bubbles  before  it  is  worked  into  its  final 
form.     This  freeing  or  "fining"  process  is  carried  out  by 
further  and  more  intense  heating  of  the  molten  glass,  which 
is  thereby  rendered  more  fluid  and  allows  the  bubbles  to 
disengage  themselves  by  rising  to  the  surface.     This  occurs 
much  more  readily  when  the  bubbles  are  large  ;  very  minute 
bubbles,  in  fact,  show  no  inclination  to  rise  through  the 
fluid  mass.     The  glass-maker  accordingly  compounds   his 
mixtures  of  raw  materials  in  such  •  a  way  as  to  yield  large 
bubbles,  or,  failing  that,  he  adds  to  the  molten  mass  some 
substance  that  evolves  a  great  many  large  bubbles,  and 
these  in  their  upward  course  through  the  glass  sweep  the 
small  ones  away  with  them.     The  added  substance  may  be 
an    inorganic    volatile    body,    such    as   arsenic,    or   more 
frequently    some    vegetable    substance    containing    much 
moisture   is   introduced  into  the  glass.     The  most  usual 
method  is  to  place  a  potato  in  the  crook  of  a  forked  iron  rod 
and  then  to  dip  the  rod  with  the  attached  potato  into  the 
molten   glass  ;    the  heat  at  once  begins  to  drive  off  the 
moisture  and  to  decompose  the  potato,  so  that  there  is  a 
violent  ebullition  of  the  whole  mass.     This  "boiling  up" 
process  assists  the  fining  considerably  and  also  serves  to 
mix  the  whole  contents  of  the  pot  very  thoroughly,  but  it 
has  some  attendant  disadvantages,  such  as  the  introduction 
of  oxide  of  iron  into  the  glass  from  the  rod  which  is  used  in 
the  operation,  while  the  contaminated  material  adhering  to 
the  walls  of  the  pot  itself  is  dragged  off  and  mixed  with  the 
rest  of  the  glass  by  the  violent  stirring  action  that  takes 
place.     It  is,  of  course,  further  obvious  that  this  process  can 
only  be  usefully  applied  to  glass  melted  in  pots,  since  the 
bulk  of  the  molten  glass  in  a  tank  furnace  could  not  be 
G.M.  G 


82  GLASS  MANUFACTURE. 

reached  at  all  in  this  manner.  Mixtures  that  are  to  be 
melted  in  tanks  must  therefore  be  capable  of  freeing  them- 
selves of  their  enclosed  bubbles  without  such  outside  aid. 
In  a  tank,  in  fact,  the  whole  melting  process  proceeds  on 
somewhat  different  lines,  since  the  temperature  of  the 
furnace  is  never  intentionally  varied,  while  on  the  other 
hand  the  melting  glass  travels  down  the  furnace  into  regions 
whose  temperature  can  be  regulated  to  favour  the  various 
stages  of  the  process  that  take  place  in  each  part  of  the 
furnace.  On  the  whole,  however,  it  is  an  undoubted  fact 
that  while  the  running  of  a  pot  furnace  can  be  varied, 
within  wide  limits,  to  suit  the  requirements  of  whatever 
mixture  it  is  desired  to  melt,  in  the  case  of  tank  furnaces 
the  mixture  must  be  closely  adjusted  to  the  requirements 
of  the  furnace,  whose  general  "  run  "  cannot  be  very  readily 
altered. 

The  completion  of  the  "  fining "  process  is  generally 
determined  by  taking  samples  of  the  glass  out  of  the  pot 
or  tank  and  examining  them  for  enclosed  bubbles.  Such 
samples  may  be  obtained  in  a  variety  of  ways,  the  most 
usual  method  being  to  dip  a  flat  iron  rod  just  below  the 
surface  of  the  glass  and  to  lift  it  out  vertically  upwards, 
thus  retaining  on  the  flat  surface  of  the  rod  some  of  the 
glass  that  lay  there  at  the  moment  when  the  rod  was 
immersed.  These  test  samples  or  "proofs  "  are  examined 
very  carefully,  and  if  no  trace  of  bubbles  can  be  observed 
the  glass  is  generally  regarded  as  "  fine,"  but  it  is  by  no 
means  certain  that  the  absence  of  bubbles  from  such  a 
small  sample  will  prove  that  the  whole  mass  is  free ;  that, 
however,  is  a  point  where  the  melter's  experience  enables 
him  to  judge  how  far  he  may  rely  upon  the  indications 


THE  PEOCESS   OF  FUSION.  83 

given  by  the  "proofs."  When  the  glass  is  "fine"  it 
frequently  happens  that  the  surface  of  the  molten  mass  is 
contaminated  by  specks  of  foreign  matter  floating  on  the 
glass ;  for  the  purpose  of  removing  these,  the  surface  of  all 
glass  is  skimmed  before  work  is  begun  upon  it.  This  is 
done  by  removing  the  surface  skin  of  glass  by  means  of 
suitably  shaped  iron  rods,  upon  which  small  masses  of 
molten  glass  are  first  "gathered."  Finally,  it  only 
remains  to  reduce  the  temperature  of  the  glass  from  that 
of  the  melting  and  fining  process  to  the  much  lower 
temperature  at  which  the  various  methods  of  working  the 
glass  are  carried  out.  In  pot  furnaces  this  is  accomplished 
by  lowering  the  temperature  of  the  entire  furnace,  while 
in  tank  furnaces  the  fine  glass  flows  into  the  working 
chamber  of  the  tank  which  is  always  kept  at  the  working 
temperature. 


G  2 


CHAPTEK  VI. 

PKOCESSES    USED    IN    THE    WOKKING    OF    GLASS. 

IN  the  previous  chapter  we  have  followed  in  outline  the 
process  of  fusion  and  fining  of  glass,  leaving  the  molten 
material  ready  for  working  up  into  the  final  shape.  Up  to 
that  point  the  process  is  very  similar  in  all  kinds  of  glass, 
although  the  furnaces,  pots  and  utensils  employed  vary 
considerably,  as  do  also  the  temperatures  to  which  the 
materials  are  heated  at  various  stages.  '  The  working 
processes,  however,  differ  entirely  from  one  class  of 
product  to  another,  as  obviously  the  process  employed  for 
the  production  of  a  sheet  of  plate-glass  can  have  little  in 
common  with  that  used  in  the  manufacture  of  a  wine-glass. 
On  the  other  hand,  the  modes  of  working  hot  glass  are  not 
so  numerous  as  the  products  that  are  produced,  so  that  we 
find  very  similar  appliances  and  manipulation  recurring  in 
various  branches  of  the  industry.  For  that  reason  we 
propose  to  deal  here  with  the  principal  methods  of  manipu- 
lating glass,  leaving  the  details  of  each  method  as  applied 
to  special  purposes  to  be  discussed  in  connection  with  the 
special  product  in  question. 

The  first  stage  in  the  working  of  all  glass  is  the  removal 
of  a  suitable  quantity  of  molten  glass  from  the  furnace. 
Practically  only  three  methods  are  available,  viz.,  ladling, 


PEOCESSES  USED  IN  THE  WORKING  OF  GLASS.      85 

pouring  and  gathering.  If  we  think  of  a  familiar  substance 
of  physical  properties  somewhat  resembling  those  of  glass, 
we  may  take  thick  treacle  and  suppose  it  confined  in  a  jar 
or  bottle ;  there  are  three  obvious  ways  of  extracting  it 
from  the  bottle :  we  may  ladle  it  out  with  a  spoon,  or  we 
may  pour  it  out  by  tilting  the  whole  bottle,  or  we  may  dip 
a  spoon  or  fork  into  the  thick  liquid,  slowly  draw  it  out 
and  turn  it  round  as  we  do  so,  thus  bringing  out  on  the 
spoon  or  fork  a  round  adherent  mass  or  "  gathering  "  of 
treacle.  In  the  case  of  molten  glass,  the  process  of  ladling 
is  by  far  the  simplest,  but  it  has  certain  very  decided 
limitations  and  disadvantages.  These  arise  from  the  fact 
that  a  ladle  cannot  be  introduced  into  molten  glass  without 
contaminating  the  whole  mass  of  glass,  at  any  rate  with 
numerous  air  bubbles.  -  The  metal  of  the  ladle  carries  with 
it  a  considerable  amount  of  closely  adherent  air  which  is 
partially  detached  while  in  contact  with  the  hot  glass,  so 
that  both  the  contents  of  the  ladle  and  the  glass  remaining 
in  the  furnace  are  contaminated.  These  bubbles  might 
perhaps  be  avoided  if  hot  ladles  were  used,  but  in  that  case 
the  glass  would  adhere  to  the  surface  of  the  metal,  and 
each  ladle  would  require  laborious  cleaning  after  each  time 
that  it  was  used.  In  practice,  therefore,  ladling  is  only 
used  for  the  production  of  those  classes  of  glass  where  the 
presence  of  a  certain  number  of  air-bells  is  not  injurious, 
and  the  ladles  are  kept  cold  by  immersion  in  water  after 
each  time  of  use.  The  use  of  the  cold  ladle  has,  however, 
the  further  disadvantage  that  a  certain  quantity  of  the 
glass  withdrawn  in  the  ladle  is  very  considerably  chilled  by 
contact  with  the  cold  metal,  and  is  thus  too  stiff  to  undergo 
the  further  processes  satisfactorily — this  chilled  glass  has, 


86  GLASS  MANUFACTURE. 

therefore,  to  be  rejected  from  each  ladleful ;  this  not  only 
involves  loss  of  glass,  but  also  necessitates  the  separation 
of  this  spoilt  glass  from  the  rest. 

The  general  process  of  rolling  requires  little  treatment 
here.  Two  essentially  different  processes  are  used  ;  in  one 
the  glass  is  thrown  on  a  flat  table  and  rolled  out  by  a 
moving  roller  passing  along  the  table ;  in  the  other  the 
glass  passes  between  two  moving  rollers,  and  the  sheet  so 
formed  is  received  on  a  moving  table  or  slab.  The  former 
mode  of  rolling  is  used  for  the  production  of  the  ordinary 
rolled  plate  glass  ;  if  the  surface  of  both  table  and  roller 
is  smooth,  the  glass  also  has  a  comparatively  smooth 
surface,  but  the  surface  is  far  from  being  level  or  free  from 
irregularities.  It  has  been  found  that  it  is  quite  impossible 
to  prevent  these  irregularities,  which  appear  to  arise  from 
the  buckling  of  the  glass  against  the  iron  surfaces  with 
which  it  comes  into  contact ;  when  rolled,  the  glass  is  too 
stiff  to  recover  its  true,  smooth  surface  under  the  influence 
of  surface  tension,  so  that  it  retains  all  the  marks  of  roller 
and  table — nor  can  the  roller  be  made  perfectly  smooth, 
since  in  that  case  it  appears  to  slip  over  the  glass  and  does 
not  roll  it  out  properly.  All  efforts,  therefore,  to  produce 
a  glass  having  a  true  and  smooth  surface  by  direct  rolling 
.  have  failed,  and  are  likely  to  fail,  so  long  as  tables  and 
rollers  are  made  of  materials  similar  to  those  now  in  use. 
The  process  of  rolling  on  a  stationary  table  is,  however, 
used  for  the  manufacture  of  plate-glass ;  but  here  the  slab 
as  rolled  has  still  the  rough,  uneven  surface  similar  to  that 
of  ordinary  "  rolled  plate,"  and  this  is  removed  and  replaced 
by  a  true  polished  surface  by  the  mechanical  processes  of 
grinding  and  polishing.  The  second  mode  of  rolling,  i.e., 


PEOCESSES  USED  IN  THE    WOEKING  OF  GLASS.      87 

with  two  or  more  "  stationary  "  rollers  and  a  moving  table, 
is  used  for  the  production  of  rolled  plate  having  special 
surface  features  or  patterns;  the  variety  of  rolled  glass 
known  as  "  figured  rolled  plate,"  having  a  deeply  imprinted 
pattern,  is  produced  in  this  way.  This  method  requires 
much  more  complicated  mechanical  appliances,  some  of 
which  are  still  protected  by  patent  rights. 

Ladling  being  thus  limited  to  the  production  of  inferior 
kinds  of  glass,  the  better  varieties  are  dependent  upon 
either  gathering  or  pouring.  The  former  process  is  limited 
as  regards  the  quantity  of  glass  that  can  be  dealt  with  in 
one  piece,  although  surprisingly  large  quantities  can  be 
gathered  upon  a  single  pipe;  the  great  masses  of  glass, 
however,  that  are  required  for  the  production  of  modern 
polished  plate  could  not  be  handled  in  this  way,  and  the 
method  of  pouring  is  accordingly  adopted.  For  this  purpose 
either  the  pots  in  which  the  glass  has  been  originally 
melted,  or  others  specially  designed  for  this  purpose,  and 
into  which  the  molten  glass  has  been  transferred,  are 
removed  bodily  from  the  furnace  by  the  aid  of  powerful 
mechanical  appliances ;  they  are  then  carried  by  overhead 
cranes  to  the  place  where  the  glass  is  to  be  rolled  into  the 
form  of  a  plate,  and  there  the  pot  is  tilted  and  the  molten 
glass  is  allowed  to  run  out  and  to  form  a  pool  on  the  rolling 
table,  the  passage  of  the  great  roller  ultimately  rolling  the 
pool  out  into  a  sheet  much  as  dough  is  rolled  out  with  a 
rolling-pin.  This  process  is  obviously  only  possible  with 
pots  or  crucibles  of  a  suitable  size,  and  is,  moreover,  very 
destructive  to  these  pots,  since  they  are  exposed  to  such 
great  variations  of  temperature.  In  the  case  of  tank 
furnaces,  numerous  devices  have  been  patented  for  allowing 


88  GLASS  MANUFACTtJKE. 

the  glass  to  flow  out  over  a  sill  or  weir  of  suitable  size, 
ready  to  be  rolled  or  drawn  into  the  form  of  sheets  or  slabs; 
but  none  of  these  devices  have,  so  far  as  the  writer  is  aware, 
found  their  way  into  practice  ;  the  reason  for  this  probably 
lies  in  the  fact  that  it  is  not  easy  to  find  a  material  which 
will  present  a  smooth  face  to  the  outflowing  glass,  such 
materials  as  fire-clay  leading  to  contamination  from 
detached  fragments,  while  chilled  metal  leads  to  local  chilling 
of  the  glass.  Unless,  therefore,  the  various  processes  of 
drawing  glass  into  sheets  direct  from  the  furnace  undergo 
very  material  improvement,  the  laborious  process  of 
gathering  is  likely  to  retain  its  importance  even  in  the 
production  of  such  large  objects  as  sheets  of  window 
glass. 

In  its  essence  the  process  of  gathering  consists  in  intro- 
ducing into  the  glass  a  heated  iron  rod  or  tube  to  which  a 
small  quantity  of  glass  is  allowed  to  adhere  ;  rod  and  glass 
are  removed  from  the  furnace  together,  and  the  small 
adherent  ball  of  glass  is  allowed  to  cool  so  far  as  to  become 
stiff  enough  to  carry  its  own  weight.  The  rod  with  its 
adherent  ball  is  then  again  dipped  into  the  glass,  where  a 
fresh  layer  of  glass  attaches  itself  to  the  ball  already  on 
the  rod.  The  whole  is  again  withdrawn,  allowed  to  cool 
down,  and  then  dipped  into  the  molten  glass  again  to 
gather  a  fresh  quantity.  This  cycle  of  operations  is  repeated 
until  the  desired  quantity  of  glass  is  attached  to  the  rod  or 
tube.  These  operations,  particularly  when  weights  of  thirty 
or  forty  pounds  of  glass  have  to  be  gathered,  require  the 
exercise  of  a  great  deal  of  skill  and  care  ;  the  introduction 
of  the  gathering  into  the  molten  glass  is  each  time  liable 
to  produce  air  bells  which  would  spoil  the  whole  mass  of 


PROCESSES  USED  IN  THE  WORKING  OF   GLASS.      89 

glass  or  would  contaminate  the  contents  of  the  crucible, 
while  subsequently  the  mass  of  hot  glass  adhering  to  the 
rod  or  pipe  tends  to  run  down  and  even  to  drop  off  entirely 
if  not  properly  checked  by  suitable  rotation  of  the  pipe. 
Further,  the  manual  labour  and  exposure  to  heat  involved 
for  the  operator  all  tend  to  increase  the  cost  of  such  work. 
Mechanical  aids  have  been  invented,  and  some  of  these  are 
in  actual  use,  but  they  are  chiefly  confined  to  mechanism 
for  relieving  the  operator  of  the  great  weight  of  the  gathering 
in  its  later  stages. 

Just  as  ladling  is  nearly  always  preliminary  to  rolling, 
so  gathering  is  usually  the  preliminary  to  some  blowing 
process,  although  the  blowing  is  often  combined  with  and 
sometimes  replaced  by  the  mechanical  pressing  of  the  glass. 
Where  the  glass  is  to  be  blown,  the  gathering  is  always 
made  on  a  glass-maker's  pipe.  This  is  an  iron  tube  from 
4  to  6  ft.  long,  provided  at  one  end  with  a  wooden  casing 
to  serve  as  a  handle,  and  with  a  suitably  arranged  mouth- 
piece for  blowing.  The  shape  of  the  lower  or  "  butt "  end 
of  the  pipe  depends  upon  the  character  and  size  of  the 
objects  to  be  blown ;  for  small  articles  the  pipe  must  be 
narrow  and  light,  but  for  heavy  sheet-glass  the  butt  of  the 
pipe  is  extended  into  a  conical  mass  whose  base  is  from 
2  to  3  in.  in  diameter.  The  bore  of  the  pipe  at  both  ends 
also  depends  upon  the  class  of  work  for  which  it  is  intended. 
The  first  stage  of  all  blowing  processes  consists  in  the 
formation  of  a  hollow  sphere  by  blowing  into  the  pipe,  the 
pressure  of  the  breath  being  as  a  rule  sufficient  to  cause 
the  gradual  distension  of  the  hot  mass  of  glass.  From  this 
rudimentary  hollow  sphere  the  various  shapes  of  blown 
articles  are  then  evolved  by  a  series  of  manipulations  which 


90  GLASS  MANUFACTUKE. 

vary  very  widely  in  different  branches  of  manufacture. 
They  generally  consist,  however,  in  gradually  changing  the 
shape  of  the  mass  of  glass  by  the  pressure  either  of  hand 
tools  or  of  specially  prepared  moulds  or  blocks  against 
which  the  glass  is  held  or  turned,  either  with  or  without 
simultaneous  blowing  into  the  pipe.  The  extent  to  which 
the  aid  of  such  moulds  and  blocks  is  invoked  varies 
continuously  from  the  production  of  the  hand -made  vase 
or  glass  to  the  moulded  bottle  ;  in  the  former,  practically 
only  hand  tools,  whose  shape  bears  no  direct  resemblance 
to  that  of  the  finished  article,  are  employed,  while  in  the 
latter  the  elongated  hollow7  mass  of  glass  is  placed  inside  a 
mould,  and  internal  air  pressure  is  used  to  press  the  glass 
into  contact  with  the  mould  from  which  the  shape  of  the 
finished  bottle  is  thus  directly  derived. 

The  art  of  the  blower  further  takes  the  fullest  advantages 
of  the  peculiar  physical  properties  of  glass  while  in  the 
heated  viscous  condition,  the  material  being  made  to  flow 
under  the  action  of  gravity  and  centrifugal  forces,  as  well 
as  under  the  pressure  of  the  breath,  the  glass  being  held 
aloft,  twirled  or  swung  about  to  ensure  the  production  of 
the  various  shapes  required.  For  the  great  majority  of  such 
purposes  the  unaided  manipulations  of  the  operator  are 
sufficient,  but  various  mechanical  aids  are  used  to  facilitate 
the  more  laborious  stages  of  the  work,  while  for  the  simpler 
forms  that  are  required  in  very  great  numbers,  such  as 
bottles,  the  whole  of  the  operations  are  now  carried  out  by 
automatic  machines.  Of  the  more  usual  mechanical  aids 
at  the  disposal  of  the  glass-blower,  we  have  already 
mentioned  hand-tools,  blocks,  and  moulds  of  various  kinds. 
Next  in  importance  to  these  is  the  use  of  compressed  air 


OF 


PEOCESSES  USED  IN  THE  WOKKING  OF  GLASS.      91 

for  blowing  large  or  heavy  articles ;  the  pressure  available 
by  the  human  breath  is  very  limited,  and  the  volume  of 
air  that  can  be  thus  delivered  is  not  very  large,  while  the 
constant  use  of  the  lungs  for  such  a  purpose  is  trying  for 
the  workman.  In  many  works,  therefore,  air  under  pres- 
sure is  supplied  to  the  benches  or  stages  where  the  blowing 
is  done,  and  the  blowers'  pipes  can  be  coupled  to  this  air- 
supply  by  means  of  flexible  connections  when  required. 
The  principal  difficulty  lies  in  the  correct  regulation  of  the 
air-pressure  for  each  special  purpose ;  but  this  difficulty 
has  been  overcome  by  the  use  of  delicate  valves  under  the 
control  of  each  blower,  who  can  thus  regulate  the  pressure 
to  his  own  exact  requirements.  Such  a  system,  of  course, 
requires  some  little  practice  on  the  part  of  the  men  using 
it,  but  when  they  have  become  accustomed  to  the  working 
of  the  plant  the  results  achieved  are  decidedly  better  and 
more  regular  than  those  obtained  by  mouth  blowing. 
Besides  the  use  of  compressed  air  supplied  in  the  way  just 
indicated,  several  other  devices  are  in  use  to  aid  the  blower 
in  producing  the  requisite  pressure  in  the  interior  of  the 
hollow  bodies  he  is  producing.  The  simplest  of  all  these 
consists  in  utilising  the  expansive  force  of  the  air  enclosed 
in  the  hollow  body  when  that  body  is  exposed  to  heat. 
Thus,  for  instance,  in  blowing  a  cylinder  of  sheet-glass,  if 
the  blower  holds  his  thumb  over  the  aperture  of  his  pipe, 
and  brings  the  closed  end  of  the  cylinder  near  the  hot 
"blowing  hole,"  the  heat  which  softens  that  end  of  the 
glass  will  also  act  upon  the  enclosed  air,  and  will  very 
rapidly  produce  such  an  expansive  effect  as  to  burst  open 
the  softened  end  of  the  cylinder,  and  this  means  of  opening 
the  closed  ends  of  the  cylinder  is  frequently  employed  in 


92  GLASS  MANUFACTURE. 

practice.  It  is,  of  course,  obvious  that  any  other  expansive 
fluid  might  be  employed  in  a  similar  manner,  and  in  some 
blowing  processes  it  has  long  been  the  practice  to  introduce 
a  small  quantity  of  water  into  the  interior  of  the  hollow 
body,  when  the  rapid  expansion  of  the  steam  produced 
thereby  is  utilised  for  the  purpose  of  generating  the 
requisite  internal  pressure.  This  use  of  the  expansive 
force  of  steam  generated  by  the  heat  of  the  hot  glass  body 
has  received  great  development  at  the  hands  of  Sievert  in 
Germany,  whose  process  is  described  in  Chapter  VII. 

Whatever  mechanical  aids  are  employed  to  facilitate  the 
various  stages  of  the  process,  all  glass  blowing  involves  a 
series  of  operations  requiring  considerable  skill,  while  the 
whole  manner  of  dealing  with  the  glass  is  essentially 
extravagant  of  material,  except  perhaps  in  the  production 
of  bottles  or  flasks  having  narrow  mouths.  The  reason  for 
this  latter  statement  lies  in  the  fact  that  by  blowing  it  is 
only  possible  to  produce  closed  or  nearly  closed  hollow 
bodies  or  vessels  ;  thus  a  blown  wine-glass  or  tumbler  is 
formed  with  a  hood  or  dome  closing  in  the  open  top  of 
the  glass,  and  this  hood  or  dome  has  subsequently  to  be 
removed  by  subsidiary  processes,  such  as  cutting  off  by  the 
aid  of  strong  local  heat  or  by  grinding,  and  the  cut  edge 
has  to  be  provided  with  a  smooth  finish.  In  the  case  of 
comparatively  small  articles  like  glasses  the  loss  involved 
from  this  cause  is  not  so  very  great,  but  were  large  flat 
bowls  or  dishes  to  be  produced  by  blowing,  the  loss  in  the 
dome  or  covering  would  be  very  serious.  This  difficulty  is 
entirely  avoided  by  the  process  of  pressing  glass.  We  have 
already  indicated  the  manner  in  which  moulds  are  used  for 
the  production  of  the  desired  shape  in  the  case  of  bottles, 


PKOCESSES  USED  IN  THE  WOBKING  OF  GLASS.      93 

etc.,  but  in  these  cases,  where  the  final  object  is  to  be  a 
hollow  vessel,  the  glass  is  readily  forced  into  contact  with 
the  mould  by  means  of  internal  air — or  steam — pressure  ; 
in  the  process  to  which  we  are  now  referring,  however,  the 
hot  glass  is  forced  into  contact  with  the  external  mould  by 
means  of  an  internal  plunger  which  is  forced  downward 
with  considerable  force.  By  this  means,  flat  or  shallow 
bodies  can  be  produced  without  the  preliminary  formation 
of  a  completely  closed  vessel,  while  it  is  obvious  that  by 
the  use  of  suitable  moulds,  complicated  and  elaborate 
shapes  can  be  produced.  It  is  true,  of  course,  that 
pressed  articles  do  not  show  the  same  smooth  and  brilliant 
surface  which  is  characteristic  of  the  fire-polish  of  blown 
articles,  while  the  facility  with  which  elaborate  surface 
ornamentation  can  be  applied  by  this  process  has  not 
tended  to  artistic  refinement  in  design,  but  the  great 
majority  of  cheap  and  useful  glass  articles  of  domestic  use 
have  been  made  available  by  the  development  of  the 
pressing  industry. 

In  the  ordinary  course,  pressed  glass  is  produced  direct 
from  the  molten  material,  which  is  introduced  into  the 
presses  either  by  gathering  or  by  means  of  ladles,  but  for 
some  special  purposes  glass  is  brought  into  its  final  shape 
by  mechanical  pressure  after  having  first  been  allowed  to 
solidify  and  having  then  been  specially  re-heated  to  undergo 
the  pressing  or  moulding  process.  This  is  principally  done 
in  the  case  of  the  best  kinds  of  optical  glass,  where  the 
molten  glass  is  first  allowed  to  cool  in  the  actual  crucible 
and  is  then  broken  up  into  lumps  of  a  suitable  size,  from 
which  the  more  defective  portions  can  be  rejected,  the 
more  perfect  portions  only  being  heated  up  again  in  special 


94  GLASS  MANUEACTUKE. 

kilns  and  then  forced  to  take  the  desired  shape  by  being 
pressed — sometimes  with  hand  tools  only  and  sometimes 
by  the  aid  of  powerful  presses — into  moulds  of  the  required 
shape.  Small  lenses,  however,  for  which  the  requirements 
of  quality  are  not  so  high  are  sometimes  pressed  direct 
from  small  gatherings  taken  from  the  molten  glass  in  the 
crucible. 


CHAPTEE    VII. 

BOTTLE    GLASS. 

ALTHOUGH  bottles  are  in  some  respects  the  cheapest  and 
crudest  products  that  are  manufactured  of  glass,  their  uses 
are  so  innumerable  and  their  numbers  so  enormous  that 
their  production  constitutes  a  most  important  branch  of 
the  industry. 

In  the  choice  of  raw  materials  for  the  production  of 
ordinary  bottles  cheapness  is  necessarily  the  first  considera- 
tion. Natural  minerals,  bye-products  of  other  industries, 
and  the  crudest  chemicals  are  utilised  so  long  as  it  is 
possible  by  compounding  these  ingredients  in  suitable  pro- 
portions to  obtain  a  glass  whose  composition  meets  the 
somewhat  crude  requirements  which  bottles  are  expected 
to  meet.  The  most  essential  of  these  requirements  are 
that  the  bottles  shall  be  strong  enough  to  resist  the  internal 
pressure  which  may  come  upon  them  when  used  for  the 
storage  of  fermented  or  effervescent  liquors  as  well  as  the 
shock  of  ordinary  use,  while  the  glass  itself  must  possess 
sufficient  chemical  resistance  to  remain  unattacked  by  the 
more  or  less  corrosive  liquids  which  it  is  called  upon  to 
contain.  Further,  from  the  point  of  view  of  the  bottle 
manufacturer  it  is  desirable  that  the  glass  shall  be  readily 
fusible,  easily  worked,  and  easily  annealed.  In  other 


96  GLASS  MANUFACTURE. 

branches  of  glass  manufacture  increased  fusibility  is  often 
attained  by  increasing  the  alkali  contents  of  the  glass,  but 
in  bottle  making  this  is  inadmissible,  both  on  account  of 
the   prohibitive   cost   of   alkali  and  because  an  increased 
alkali  content   renders   the  glass  more  liable  to  chemical 
attack.     On  the  other  hand,  in  many  varieties  of  bottle  the 
colour  of  the  glass  is  nearly,  or  quite,  immaterial  so  that 
the  introduction  of  relatively  large  proportions  of  iron  oxide 
is  permissible.     This  substance  acts  as  a  flux  and  assists  in 
the  production  of  a  fusible,  workable  glass  containing  little 
alkali.      Such  alkali  as  bottle  glass  does  contain  is  fre- 
quently derived  from  felspathic  minerals,  which  generally 
also  contain  considerable  proportions  of  iron.     The  use  of 
these   minerals   also    introduces    notable    proportions    of 
alumina    into   the   glass.      In   certain   classes   of   bottles, 
notably  those   used   for   special  wines,   certain   shades  of 
colour  are  required — the  well-known  "  Hock  bottle  "  colour 
being  an  example.     The  presence  of  iron  in  the  glass  tends 
to  the  production  of  a  green  or   greenish   yellow  colour 
deepening  to  a  black  opacity  if  the  quantity  of  iron  be  high. 
The  lighter  shades  of  this  green  tint  may  be  "  neutralised  " 
by    the   introduction    of    manganese    into    the   glass,    the 
resulting   colours    ranging   from   light    amber   to   purple ; 
nickel  oxide  is  also  sometimes  used  as  a  colouring  material 
in  these  glasses. 

In  the  production  of  ordinary  bottles  the  continuous 
tank  furnace  has  now  entirely  superseded  the  old  pot 
furnaces,  the  character  of  the  product  being  in  this  case  par- 
ticularly suited  to  this  process  of  production.  The  modern 
bottle-glass  tank  is  generally  an  oblong  basin  having  one 
semi-circular  end.  The  flame  is  often  of  the  "  horse-shoe  J> 


BOTTLE   GLASS.  97 

type,  the  gases  both  entering  and  leaving  the  furnace  at 
the  flat  or  charging  end  of  the  furnace.  The  raw  materials 
are  thrown  into  the  furnace  at  the  square  end  of  the  tank, 
and  the  glass  flows  uninterruptedly  down  the  furnace  to 
the  colder  semi-circular  end  where  the  working  holes  are 
situated.  At  these  points  fire-clay  rings  are  kept  floating 
on  the  glass,  and  from  within  these  the  gatherer  takes  his 
gathering,  the  rings  serving  to  retain  the  grosser  impurities 
carried  down  by  the  glass.  The  producing  power  of  such 
a  furnace,  even  when  the  bottles  are  blown  by  hand,  is 
very  considerable ;  a  furnace  having  ten  working  holes 
and  containing  normally  about  85  tons  of  molten  glass 
will  yield  some  four  million  bottles  per  annum,  and  furnaces 
of  considerably  larger  capacity  are  in  use. 

The  methods  of  bottle  making  are  at  the  present  time 
passing  through  what  is  probably  a  stage  of  transition. 
Up  to  the  middle  of  last  century  the  processes  in  use  were 
little  better  than  those  of  the  middle  ages  ;  the  first  step  of 
a  more  modern  development  of  the  industry  took  the  direc- 
tion of  improved  tools  and  implements  for  carrying  out  the 
old  operations.  More  recently  a  whole  series  of  inventions 
have  been  put  forward  with  the  aim  of  producing  bottles  by 
entirely  different  and  wholly  mechanical  processes  with  the 
object  of  eliminating  the  uncertain  element  of  skilled  labour 
entirely.  While  it  must  be  admitted  that  some  of  the 
earlier  of  these  inventions  proved  to  be  brilliantly  ingenious 
failures,  there  is  little  doubt  that  here,  as  in  other  manu- 
facturing processes,  the  machine-made  article  will  ultimately 
supersede  the  hand-made  product.  Even  now,  mechanical 
processes  are  largely  in  use  both  in  America  and  Europe, 
and  at  some  recent  exhibitions  machine-made  bottles  have 

G.M.  H 


98  GLASS  MANUFACTUEE. 

been  shown  which  in  every  point  of  quality  were  superior 
to  the  best  hand-made  goods. 

The  first  stage  in  the  production  of  bottles  by  hand,  and 
also  for  most  of  the  machine  processes,  is  that  of  gathering 
the  requisite  quantity  of  glass.  The  bottle-blower's  pipe  is 
between  5  and  6  ft.  long,  and  is  provided  with  a  slightly 
enlarged  end  or  "  nose  "  upon  which  the  glass  is  gathered. 
Three  gatherings  are  generally  sufficient  for  the  production 
of  ordinary  bottles,  but  for  extra  large  bottles,  and  especi- 
ally for  carboys,  heavier  gatherings  are  necessary,  and  for 
these  the  gatherer  must  go  to  the  furnace  four,  five,  or  even 
six  times.  When  the  requisite  quantity  of  glass  has  been 
gathered  on  the  pipe  the  gathering  is  worked  and  rounded 
by  rolling  it  either  on  a  flat  metal  plate  or  "marver,"  or 
in  a  hollowed  block  made  of  wood  or  more  rarely  of  metal ; 
by  this  process  the  glass  is  formed  into  a  well-rounded, 
symmetrical  pear-shaped  body.  The  blower  now  distends 
the  mass  gradually  by  the  pressure  of  his  breath,  at  the 
same  time  swinging  the  pipe,  the  effect  of  these  movements 
being  to  draw  the  bulk  of  the  glass  downwards,  leaving  a 
thinner  and  colder  portion  having  the  rudimentary  shape 
of  the  neck  of  the  bottle  next  to  the  pipe.  In  the  oldest 
form  of  the  process  the  next  stage  in  the  production  of  the 
bottle  is  accomplished  by  the  aid  of  a  cylindrical  mould  of 
fire-clay,  whose  diameter  is  that  of  the  external  size  of  the 
finished  bottle.  The  pear-shaped  bulb  of  glass  is  for  this 
purpose  re-heated  at  the  melting  furnace,  and  is  then  placed 
inside  the  fire-clay  mould.  By  vigorous  blowing,  and  a 
rapid  rotation  of  the  pipe  and  glass,  the  bulb  is  forced  to 
assume  the  cylindrical  shape  of  the  mould,  the  glass  form- 
ing the  neck  of  the  bottle  being  at  this  stage  of  the  process 


BOTTLE  GLASS.  99 

too  cold  and  stiff  to  be  further  deformed.     The  next  step  is 
the  formation  of  the  concavity  found  in  the  base  of  wine 
and  beer  bottles ;  this  is  produced  by  pushing  up  the  hot 
plastic  glass  that  forms  the  bottom  of  the  bottle  as  it  leaves 
the  clay  mould.      This  is  done  by  a  second  workman  using 
an  iron  rod  known  as  the  "  pontil,"  upon  which  a  small  mass 
of  glass  has  previously  been  gathered.      This  mass  of  glass 
remains  attached  to  the  bottom  of  the  bottle,  which  is  thus 
for  the  moment  fastened  both  to  the  "pontil "  and  to  the 
blower's  pipe.     The  blower,  however,  immediately  detaches 
the  bottle  from  the  pipe  at  the  point  where  the  neck  of  the 
bottle  is  intended  to  end,  effecting  this  by  locally  chilling 
the  glass — a  process  known   by  the   descriptive   term   of 
"  wetting  off."     The  unfinished  bottle  is  now  attached  to 
and  handled  by   means  of   the    "  pontil."      The  neck   is 
softened   by   re-heating   it  over  the  furnace,  and  is  then 
moulded  into  the   desired  shape  by  the  aid  of  specially- 
shaped  tongs.     Finally  a  thread  of  glass  is  wound  round 
the   end  of  the  neck  to  produce  the  thickening   usually 
found  at  that  point.     The  finished  bottle,  still  attached  to 
the  "pontil,"  is  now  carried  to  the  annealing  kiln,  where 
it  is  placed  in  position  and  detached  from  the  "  pontil "  by 
a  sharp  blow,  which  severs  the  glass  that  had  been  gathered 
on  the  "  pontil  "  from  the  bottom  of  the  bottle. 

The  process,  in  the  form  described  above,  has  been  obso- 
lete for  many  years,  improvements,  consisting  of  appliances 
for  facilitating  the  various  operations,  having  been  gradually 
introduced.  The  most  important  of  these  is  the  substitu- 
tion of  metal  moulds  for  the  fire-clay  moulds  of  earlier 
times.  These  metallic  moulds  are  made  to  open  and  close 
at  will  by  the  action  of  a  pedal,  and  are  designed  to  give  the 

E  2 


100  GLASS  MANUFACTUKE. 

entire  bottle  its  final  shape,  except  for  the  indentation  of  the 
bottom,  although  this  is  sometimes  produced  by  a  convex 
piece  placed  on  the  bottom  of  the  mould.  In  the  forma- 
tion of  the  neck  thickening,  also,  important  mechanical  aids 
have  become  almost  universal.  These  last  consist  of  tongs 
provided  with  rollers  and  arranged  to  rotate  about  an  axis 
that  terminates  in  a  tapered  spike  which  enters  the  neck  of 
the  bottle ;  by  pressing  the  tongs  together  so  as  to  bring 
the  rollers  against  the  outside  of  the  neck  and  rotating  the 
whole,  the  rollers  are  made  to  form  the  neck  thickening  in 
an  accurate  and  rapid  manner. 

Important  and  valuable  as  these  improvements  of  the 
ancient  process  of  bottle-blowing  undoubtedly  are,  they  do 
not  touch  the  main  disadvantages  of  the  process — dis- 
advantages that  seriously  affect  the  economy  of  the  process 
and  the  well-being  of  the  workers  employed  upon  it.  It  is 
consequently  not  surprising  that  a  great  number  of  inventors 
have  laboured  at  the  problem  of  the  purely  mechanical 
production  of  bottles.  A  large  number  of  patents  have 
accordingly  been  taken  out  in  connection  with  bottle-making 
machinery.  The  first  of  these  to  attain  any  favour  was 
that  devised  by  Ashley,  but  although  great  claims  were 
made  for  it,  its  use  has  not  extended.  At  the  present  time, 
however,  there  are  a  number  of  bottle-works  actually  at 
work  producing  bottles  by  mechanical  means ;  one  of  the 
most  successful  of  these  machines  is  that  devised  by 
Boucher,  of  Cognac.  The  products  of  this  machine, 
exhibited  in  Paris  at  the  exhibition  of  1900,  were  equal, 
and  possibly  superior,  to  the  best  hand-made  bottles.  The 
Boucher  machine,  although  by  no  means  entirely  auto- 
matic, requires  no  highly-skilled  labour  beyond  that  of  a 


BOTTLE   GLASS.  101 

workman  whose  duty  it  is  to  operate  the  various  levers  of 
the  machine  at  the  right  instant  and  in  the  proper  order. 

The  details  of  the  machine,  as  set  forth  in  the  patents 
and  other  published  descriptions,  are  somewhat  complicated, 
and  vary  somewhat  in  the  different  models  ;  the  general 
principle  and  mode  of  operation  is,  however,  the  same  in  all 
varieties  of  the  machine,  and  we  shall  therefore  give  a  brief 
account  of  it  here. 

In  the  Boucher  process,  the  glass  is  first  gathered  from 
the  furnace,  but  as  no  blowing-pipes  are  required,  the 
gathering  is  done  on  a  light  iron  rod,  thus  saving  the 
gatherer  much  of  the  labour  of  carrying  the  heavy  pipes. 
The  requisite  quantity  of  the  glass  so  gathered  is  then  dropped 
into  the  first  or  "measuring"  mould  of  the  machine,  the 
"  thread  "  being  cut  by  hand  by  the  operator.  From  the 
measuring  mould,  the  glass  is  next  caused  to  pass  into  the 
"neck"  mould;  the  glass  flows  into  this  mould,  and  is 
further  pressed  into  it  by  the  aid  of  compressed  air,  applied 
above  the  free  surface  of  the  glass.  At  this  stage  the  still 
liquid  glass  has  the  external  shape  of  the  neck  of  the  bottle, 
but  the  mass  of  glass  is  solid,  i.e.,  no  cavity  has  yet  been 
produced  in  it.  The  formation  of  the  cavity  is  next  begun 
by  the  action  of  a  plunger  which  is  driven  into  the  "  solid  " 
mass  of  glass  filling  the  neck  mould,  this  plunger  thus 
punching  out  the  passage  through  the  neck  of  the  bottle. 
As  soon  as  the  plunger  is  withdrawn,  compressed  air  is 
admitted  into  the  cavity  so  formed,  and  the  mass  of  glass  is 
at  the  same  time  inverted,  and  that  part  occupying  the 
position  of  what  is  to  be  the  shoulder  of  the  bottle  is  allowed 
to  descend  while  being  blown  out  by  the  compressed  air. 
This  process  of  distension  is  limited,  and  the  desired  shape 


102  GLASS  MANUFACTURE. 

is  imparted  to  the  mass  by  bringing  towards  it  a  third 
mould,  by  contact  with  which  the  glass  is  considerably 
stiffened — a  row  of  jets  of  compressed  air,  impinging  on  the 
outside  of  the  glass  forming  the  shoulder  of  the  bottle,  being 
further  used  to  stiffen  the  glass,  once  the  requisite  extension 
has  been  attained.  The  mass  has  now  a  shape  very  similar 
to  that  known  as  a  "  parason  "  in  hand  bottle-blowing,  and 
is  by  this  time  decidedly  stiff.  It  is  now  introduced  into 
the  finishing  mould  and  is  blown  into  perfect  contact  with 
the  mould  by  powerful  air-pressure,  thus  attaining  the 
proper  shape  of  barrel  and  base  ;  the  indentation  of  the 
base  is,  however,  sometimes  produced  on  a  separate  machine 
or  press.  During  all  these  operations  the  neck  of  the 
bottle,  which  was  the  first  part  to  be  formed,  has  remained 
firmly  held  in  the  neck  mould,  and  all  the  movements  that 
have  been  described  are  performed  by  means  of  levers 
actuating  movements  of  this  mould  as  a  whole,  which,  of 
course,  carry  the  glass  with  them.  The  last  movement  of 
the  levers,  which  releases  the  bottle  from  the  finishing 
mould,  also  opens  the  neck  mould,  and  thus  leaves  the 
bottle  finished  and  entirely  free. 

It  will  be  seen  that  the  process  adopted  in  this  machine 
follows  as  closely  as  possible  the  various  stages  of  hand 
blowing,  but  that  the  mechanical  movements  of  the  machine 
replace  the  laborious  and  difficult  technique  of  the  blower. 
One  such  machine  is  capable  of  producing  as  many  as  120 
bottles,  each  weighing  If  Ibs.,  per  hour,  but  this  is  accom- 
plished only  by  having  some  of  the  moulds  in  duplicate  and 
so  arranged  as  to  come  into  use  alternately.  The  machine 
itself  is  attended  by  one  "  moulder,"  who  operates  the 
levers,  and  by  a  youth'  who  carries  the  finished  bottles  to 


BOTTLE  GLASS.  103 

the  annealing  kiln,  while,  of  course,  the  services  of  a 
gatherer  are  also  required.  The  appearance  of  a  bottle 
works  equipped  with  these  machines  is  in  striking  contrast 
to  that  of  a  hand-blowing  works,  where  the  stages  around 
the  working-holes  are  crowded  with  men  doing  arduous 
work  under  very  severe  conditions  of  temperature  and 
atmosphere.  Finally,  it  must  be  pointed  out  that  the  use 
of  the  Boucher  machine  is  by  no  means  confined  to  the 
production  of  the  cheapest  kinds  of  bottles,  but  that  it  has 
shown  itself  especially  well  suited  to  the  production  of 
champagne  and  other  bottles  that  are  required  to  withstand 
a  high  internal  pressure,  the  machine-made  bottles  showing 
excellent  results  under  pressure  tests.  The  machine  is  also 
used  for  the  production  of  moulded  glass-ware  of  white 
glass,  since  it  can  be  adapted  to  the  production  of  any  kind 
of  glass  vessel  that  can  be  produced  by  blowing  into  a 
mould. 

The.  annealing  of  bottles  was  formerly  carried  out  in  large 
chambers  or  kilns  of  very  simple  construction,  in  which 
the  bottles  were  stacked  as  made,  the  kiln  being  previously 
heated  to  the  requisite  temperature :  when  full,  the  kiln 
was  closed  up  in  a  rough  temporary  manner  and  allowed  to 
cool  naturally,  thus  annealing  the  bottles  stacked  within  it. 
In  this  branch  of  glass-making  also,  however,  the  continuous 
annealing  kiln  has  superseded  the  older  kinds,  and  con- 
tinuous kilns  are  now  almost  universal  in  bottle-making. 
In  these  kilns,  which  consist  of  long  tunnels,  kept  hot  at  one 
end  and  having  a  gradually  decreasing  temperature  as  the 
other  end  is  approached,  the  bottles  are  stacked  on  trucks 
which  are  slowly  drawn  through  the  kiln  from  the  hot  to 
the  cold  end.  At  the  cold  end  the  trucks  are  unloaded  and 


104  GLASS  MANtJFACTtMS. 

are  then  returned,  by  an  outside  route,  to  the  charging  end, 
but  of  course  the  bottles  cannot  be  stacked  on  the  truck 
until  it  has  actually  entered  the  hot  end  of  the  tunnel  and 
acquired  the  temperature  there  prevailing.  In  a  slightly 
different  form  of  kiln,  the  bottles  are  carried  down  the  kiln 
on  a  species  of  conveyer  belt  formed  of  iron  plates,  but  the 
principle  of  all  these  appliances  is  similar  even  when  used 
for  very  different  kinds  of  glass. 

In  the  account  of  bottle  manufacture  given  above  we  have 
referred  almost  exclusively  to  the  mode  of  production  of  the 
ordinary  bottles  used  for  the  storage  of  such  liquids  as  wine, 
beer,  spirits,  etc.,  and  we  will  now  deal  with  some  other 
branches  of  manufacture  closely  allied  to  these. 

An  important  branch  of  glass  manufacture  is  the  pro- 
duction of  vessels  of  large  dimensions.  Those  most  closely 
allied  to  ordinary  bottles  are  the  vessels  known  as  carboys, 
used  for  the  storage  and  transportation  in  bulk  of  chemical 
liquids,  and  especially  of  acids.  Formerly  these  were  blown 
by  hand  in  a  manner  closely  resembling  that  used  for 
ordinary  bottles,  but  the  weight  of  the  mass  of  glass  to  be 
handled  by  gatherer  and  blower  is  very  great,  while  the 
lung-power  of  a  blower  is  not  sufficient  to  produce  the  great 
expansion  required.  Formerly  the  only  aid  available  to 
the  blower  was  the  device  of  injecting  into  the  hot,  hollow 
glass  body,  at  an  early  stage  of  the  process,  a  quantity  of 
water  or  alcohol ;  this  liquid  was  immediately  vapourised 
by  the  heat  of  the  glass,  and  if  the  blower  closed  the  mouth- 
piece end  of  his  pipe  by  placing  his  thumb  over  it,  the  expan- 
sive force  of  the  vapour  so  generated  served  to  blow  out  the 
glass  to  the  desired  extent.  More  recently  mechanical 
aids  to  the  production  of  these  large  vessels  Lave  become 


BOTTLE  GLASS.  105 

available,  first  in  the  shape  of  mechanical  arrangements  for 
relieving  the  workmen  of  the  full  weight  of  the  glass  and 
pipe  by  providing  suitable  arms  upon  which  the  whole  can 
be  supported  without  interfering  with  the  blower's  freedom 
of  manipulating  the  pipe  and  glass  in  the  desired  way ; 
further,  a  supply  of  compressed  air,  which  can  be  readily 
connected  with  the  pipe  at  any  desired  moment,  facilitates 
the  blowing  process. 

A  process  of  producing  hollow  glass  vessels  of  very  large 
size  by  purely  mechanical  means  has,  however,  been  intro- 
duced during  recent  years  by  P.  Sievert,  of  Dresden.     By 
the  methods  of  this  inventor,  glass  vessels  of  quite  unprece- 
dented size — such  as  bath-tubs  freely  accommodating  full- 
grown  men — can  be  produced.     For  this  purpose  the  glass 
is  spread  out  on  the  surface  of  a  large  cast-iron  plate,  pro- 
vided with  numerous  small  holes  through  which  steam  or 
compressed  air  may  be  blown  when  desired.     The  slab  of 
viscous   glass,    when   properly  spread    over  this  plate,  is 
clamped  down  against  it  all  around  the  outside  edge  by 
means  of  a  suitably-shaped  iron  collar,  which  holds  the 
glass  in  air-tight  contact  against  the  plate  beneath.     The 
whole  iron  plate,  with  the  slab  of  glass  clamped  to  it,  is 
now  turned  over,  so  that  the  glass  hangs  down  under  the 
plate.     The  glass  immediately  begins  to  sag  under  its  own 
weight,  and  is  assisted  in  this  tendency  by  a  suitable  blowing 
of  steam  or  air  into  the  space  between  the  plate  and  the 
glass.     In    blowing   bath-tubs   in   this   way   the   glass   is 
allowed  to  distend  downwards  until  the  desired  depth  is 
attained,  when  further  distension  is  arrested  by  bringing  a 
flat   supporting   plate    under  the  glass,  which  is  pressed 
against   this   flat   plate  by  the  pressure  of  the  air,  thus 


106  GLASS  MANUFACTUEE. 

forming  the  flat  bottom  of  the  tub.  In  this  process  the 
outline  of  the  object  is  determined  by  the  shape  of  the 
clamping  bars  or  plate  that  fix  the  edges  of  the  hot  glass 
against  the  iron  plate  described  above,  and  by  this  means 
almost  any  desired  shape  can  be  given  to  objects  of  simple 
form. 

It  is  obvious  that  this  process  can  also  be  employed  for 
blowing  a  hollow  body  into  contact  with  a  mould  of  any 
desired  form  and  forcing  the  hot  glass  to  take  the  exact 
shape   of    the   mould ;    for    smaller  bodies,  however,    the 
blowing  in  of  separately  generated  steam  is  not  required, 
the  heat  of  the  molten  glass  itself  being  used  to  generate  the 
necessary  steam.     For  this  purpose  the  requisite  quantity 
of  glass  is  dropped  on  the  surface  of  a  wet  slab  of  asbestos. 
On  this  surface  the  glass  remains  floating  upon  a  layer  of 
steam,  which  is  constantly  renewed  by  the  intense  heating 
action  of  the  hot  glass  on  the  water  contained  in  the  asbestos 
below.     The  moulds  used  in  this  process  are  provided  with 
a  sharp  edge  or  lip,  and  as  soon  as  the  glass  has  spread 
into  a  slab  of  sufficient  size,  the  inverted  mould  is  brought 
down  upon  the  glass  and  pressed  against  it.     The  sharp  lip 
or  edge  of  the  mould  forces  the  glass  into  close  contact  with 
the  asbestos  under  it  all  around  the  edge  of  the  mould, 
thereby  enclosing  the  space  existing  between  the  rest  of  the 
glass  and  the  wet  asbestos.     The  heat  of  the  glass  continues 
to  generate  steam  at  a  rapid  rate,  but  now  the  steam  can 
no  longer  escape  from  under  the  glass  around  the  edges, 
and   therefore   blows   the  glass  upwards  into  the  mould, 
ultimately  forcing  the  glass  into  intimate  contact  with  the 
surface   of   the   mould ;    when    this   is   accomplished,  the 
pressure  of  the  steam  rises  rapidly,  and  ultimately  lifts  the 


BOTTLE  GLASS.  107 

entire  mould  and  glass  sufficiently  to  allow  the  excess  steam 
to  escape — and  this  is  the  sign  that  the  blowing  is  complete. 
The  whole  process  takes  only  a  very  few  seconds,  and  is 
very  successful  when  applied  to  suitable  glass  and  used  with 
moulds  of  proper  shape.  It  is,  of  course,  obvious  that 
ordinary  narrow-mouthed  bottles  could  not  be  produced  in 
this  way,  but  wide-mouthed  bottles  and  jars  are  made  in 
this  manner,  although  the  chief  utility  of  the  process  lies 
in  the  production  of  comparatively  shallow  articles,  which 
are  not  of  a  shape  that  lends  itself  to  pressing. 


CHAPTEK   VIII. 

BLOWN    AND    PRESSED    GLASS. 

IN  many  ways  very  similar  to  the  processes  employed  in 
the  production  of  hottles  are  those  used  in  the  manufacture 
of  all  hollow  glass  vessels  that  are  produced  by  blowing, 
either  with  or  without  the  aid  of  moulds.  Apart  from  the 
actual  shapes  of  the  articles  themselves,  however,  the 
principal  difference  between  bottles  and  the  better  classes  of 
hollow  glass-ware  lies  in  the  composition  and  quality  of  the 
glass  itself.  In  this  respect  all  grades  of  manufacture  are 
to  be  met  with,  from  the  light-coloured  greenish  or  bluish 
glass  used  for  medicine  bottles  to  the  most  perfectly 
colourless  and  brilliant  "  crystal  "  or  flint  glass.  This 
gradation  in  the  perfection  of  the  glass  represents  a  corre- 
sponding gradation  in  the  care  bestowed  upon  the  choice  of 
raw  materials  and  the  various  manipulations  of  melting 
the  glass.  As  we  have  seen,  for  the  commonest  kinds  of 
bottles,  where  colour  and  quality  are  immaterial,  all  kinds 
of  fusible  materials  can  be  utilised,  loamy  or  ferruginous 
sands  and  refuse  glass  of  all  kinds  being  employed.  Where 
somewhat  higher  requirements  have  to  be  met,  rather 
purer  sands  have  to  be  used  as  sources  of  silica,  while  lime 
and  alkali  must  be  introduced  in  purer  forms,  the  alkali 
in  the  shape  of  the  cheapest  qualities  of  salt-cake  and  the 


BLOWN  AND  PEESSED   GLASS.  109 

lime  in  that  of  lime- stones  reasonably  free  from  iron  and 
magnesia.  Finally,  for  the  best  qualities  of  glass  the 
purest  sand  obtainable  is  used,  being  often  specially  washed 
to  remove  all  loamy  matter,  while  the  alkali  is  introduced 
in  the  form  of  carbonate,  a  chemical  product  which  in  its 
better  qualities  is  practically  free  from  injurious  impuri- 
ties. In  these  high-class  products  two  very  distinct  kinds 
of  glass  are  met  with.  One  class,  of  which  the  Bohemian 
"  crystal  "  is  the  highest  example,  is  chemically  of  the 
nature  of  an  alkali-lime  silicate,  the  alkali  in  the  case  of  the 
Bohemian  glass  being  j^otash ;  the  other  variety  of  glass 
contains  no  lime,  its  place  being  taken  by  lead,  typical  of 
this  class  being  English  flint  glass.  In  some  varieties  of 
glass,  lead  is  also  replaced,  partially  or  entirely,  by  barium, 
but  this  material  is  chiefly  used  for  the  manufacture  of 
pressed  glass. 

The  higher  grades  of  quality  in  glass,  which  thus  require 
increased  refinement  in  the  raw  materials,  also  demand 
increased  refinement  in  the  furnaces  and  appliances 
employed  in  their  melting.  The  tank-furnace,  which  holds 
the  field  in  bottle  manufacture,  is  scarcely  met  with  in  the 
production  of  the  better  qualities  of  hollow  glass-ware  ; 
medicine  bottles  and  other  articles  of  moderate  quality 
might  be  produced  in  tanks,  but  the  quantity  of  glass 
required  for  such  purposes  is  seldom  large  enough  to 
justify  such  large  plant.  For  the  best  qualities  of  colour- 
less glass-ware,  however,  the  tank-furnace  could  not  be 
used  on  account  of  the  fact  that  both  as  regards  colour  and 
freedom  from  defects,  the  product  of  a  tank-furnace  is 
never  equal  to  the  best  product  of  pot-furnaces.  For 
flint-glass,  indeed,  covered  pots  or  crucibles  must  be  used  in 


110  GLASS  MANUFACTUKE. 

order  to  adequately  protect  the  molten  glass  from  the  reduc- 
ing action  of  the  furnace  gases  and  from  contamination  by 
dust.  The  materials  of  which  the  pots  are  constructed  are 
also  chosen  with  a  view  to  avoiding  all  risk  of  introducing 
colouring  or  otherwise  injurious  impurities  from  that 
source. 

In  all  processes  for  the  production  of  hollow  glass-ware, 
the  glass  or  "  metal  "  is  taken  from  the  pot  by  the  process 
of  gathering  which  has  already  been  described;  where 
blown  articles  are  to  be  produced,  as  distinct  from  pressed 
goods,  the  initial  stage  is  always  the  formation  of  a  small 
hollow  globe  or  bulb  at  the  end  of  the  glass-blower's  pipe. 
The  subsequent  manipulations  depend  upon  the  nature  of 
the  article  to  be  produced.  The  article  may  either  be 
made  entirely  by  hand  work,  or  rather  "chair"  work,  as 
it  is  usually  called,  or  the  manipulations  may  be  facilitated 
and  the  product  cheapened — while  its  character  is,  of  course, 
also  modified — by  the  aid  of  moulds,  which  are  used  to 
bring  the  object  to  its  proper  shape  and  to  impress  upon  it 
certain  decorative  mouldings  or  markings.  As  we  have 
already  seen,  ordinary  bottles  are  now  always  blown  with 
the  aid  of  moulds,  and  the  same  applies  to  medicine 
bottles,  lamp  chimneys,  and  the  bulbs  for  electric  light ; 
in  connection  with  lamp-chimneys  it  should  be  noted  that 
they  are  blown  in  moulds  in  the  form  of  cylindrical  bottles 
with  a  flat  bottom  and  a  domed  top,  the  ends  being 
subsequently  cut  off. 

Many  of  the  cheaper  varieties  of  tumblers  and  glasses 
are  also  blown  in  moulds,  but  they  can  be,  and  sometimes 
are,  produced  by  hand,  and  as  their  manufacture  is  typical 
of  that  of  all  hand-blown  hollow  ware,  we  shall  now 


BLOWN  AND  PRESSED  GLASS.  Ill 

describe  it  in  some  detail  as  an  example  of  this  class  of 
work. 

The  implements  used  by  the  glass-blower  and  his  assis- 
tants for  this  work  are  few  and  simple.  The  largest  item 
is  the  glass-blower's  bench  or  chair,  which  is  simply  a 
rough  wooden  bench  provided  with  two  projecting  side- 
rails  or  arms.  When  finishing  a  piece  of  work  the  blower 
sits  on  this  bench,  and  the  pipe  lies  across  the  two  rails  in 
front  of  him  in  such  a  position  that  by  rolling  it  backwards 
and  forwards  along  the  rails  he  can  readily  keep  the  pipe  in 
gentle  rotation.  In  addition  to  the  ordinary  blower's  pipe 
and  a  "  pontil  "  or  rod  for  attaching  small  quantities  of 
glass  whereby  the  piece  in  hand  can  be  held,  the  only  other 
tools  used  by  the  blower  are  a  number  of  shears  and 
pincers  of  various  shapes  which  serve  for  cutting  off,  press- 
ing in,  and  distending  the  glass  as  required,  a  flat  board 
and  a  stone  or  metal  plat  or  "  marver  "  being  also  used  for 
the  purpose  of  moulding  the  glass. 

As  already  indicated,  the  first  step  in  the  production  of 
such  an  object  as  a  tumbler  consists  in  gathering  a  suitable 
quantity  of  glass  on  the  pipe  and  blowing  it  into  a  small 
bulb.  This  bulb  is  blown  out  to  the  proper  size  and  is  then 
elongated  by  gently  swinging  the  pipe.  The  next  step  is 
the  flattening  of  the  lower  end  of  the  bulb  by  gently  press- 
ing it  on  the  "  marver  "  or  flat  plate  provided  for  such 
purposes ;  in  this  way  the  flat  bottom  of  the  glass  is  formed, 
and  the  bulb  now  has  the  shape  of  the  finished  glass,  but 
remains  attached  to  the  pipe  by  a  shoulder  and  neck.  The 
earliest  practice  was  to  separate  the  tumbler  from  the  pipe 
at  such  a  point  as  to  leave  the  tumbler  of  the  correct  length, 
the  remaining  operation  consisting  in  holding  the  glass, 


112  GLASS  MANUFACTURE. 

first  fixed  to  a  pontil  for  the  purpose,  into  the  furnace  so  as 
to  heat  the  broken  edge ;  this  edge  was  thereby  rounded  off, 
and  the  brim  of  the  glass  could  be  widened  or  otherwise 
shaped  by  rotating  the  glass  or  pressing  it  in  or  out  by  the 
aid  of  pieces  of  wood.  In  modern  practice,  however,  this  is 
not  usual,  the  glass  being  separated  from  the  pipe  well 
above  the  shoulder  and  annealed  in  this  shape.  Subse- 
quently the  glass  is  finished  in  a  trimming  room  or  work- 


FIG.  8. — Sectional  diagram  of  the  evolution  of  a  tumbler. 

shop  by  being  cut  off  at  the  desired  point  and  having  the 
rough  edge  rounded  off  by  the  aid  of  a  blowpipe  flame. 
The  cutting-off  operation  is  carried  out  in  a  great  variety 
of  ways,  the  most  usual  being  by  the  action  of  heat  applied 
locally  and  suddenly,  either  by  the  aid  of  specially-shaped 
flat  blowpipe  flames  or  by  an  electrically-heated  wire. 
Machines  for  carrying  out  this  operation,  as  well  as  the 
subsequent  rounding  of  the  edge  automatically,  are  in  use, 
but  the  latter  process  is  sometimes  replaced  by  slightly 
grinding  and  polishing  the  edges. 


BLOWN  AND  PEESSED  GLASS.  113 

The  evolution  of  an  ordinary  tumbler,  as  just  described, 
and-  as  illustrated  diagrammatically  in  Fig.  8,  is  typical  of 
the  whole  process  of  hollow-glass  blowing,  but  of  course  the 
number  of  operations,  as  well  as  the  care  and  skill  involved 
in  each  step,  increases  rapidly  as  the  form  of  the  vessel 
becomes  more  complex  ;  in  the  highest  class  of  work  a  very 
considerable  element  of  artistic  taste  and  judgment  on  the 
part  of  the  operative  also  becomes  essential,  for,  although 
the  form  of  the  object  as  well  as  the  choice  of  colour  and 
ornamentation  are  chosen  by  the  designer,  the  blower  has 
to  translate  the  drawing  of  the  designer  into  glass,  and 
although  his  skill  enables  him  to  attain  a  considerable 
degree  of  fidelity  in  his  rendering,  many  details  remain  at 
his  own  option,  and  the  proper  management  of  these  is  no 
small  factor  in  the  success  of  the  whole  work. 

In  this  connection  mention  should  perhaps  be  made  of 
the  application  of  colour  and  other  decorations  to  this  kind 
of  glass.  A  very  considerable  range  of  effects  of  this  kind 
is  now  available  to  the  glass-worker.  In  the  first  place  the 
body  of  the  glass  used  for  the  production  of  the  articles  in 
question  may  be  coloured  by  the  addition  of  suitable 
colouring  materials  to  the  molten  glass  or  raw  materials,  as 
explained  in  Chapter  XI.,  but  this  procedure  has  very  obvious 
limitations ;  where  the  article  is  built  up  of  glass  from 
several  gatherings — as,  for  example,  is  the  case  in  an 
ordinary  wine-glass,  where  the  bowl,  leg  and  foot  are  each 
made  of  separate  gatherings — it  is  possible  to  use  glass  of 
different  colours  for  these  different  parts,  and  this  is 
commonly  done  in  the  production  of  wine  glasses  having 
ruby  or  green  bowls  and  white  legs  and  feet.  A  further 
modification  in  the  application  of  colour  is  obtainable  by 
G.M.  T 


114  GLASS  MANUFACTURE. 

taking  up  two  or  more  gatherings  on  the  same  pipe  and 
superposing  a  large  gathering  of  white  glass  on  a  smaller 
one  of  coloured  glass;  this  is  analogous  to  the  process  of 
"flashing"  sheet  glass,  described  in  Chapter  X.  and  this 
process  lends  itself  to  a  variety  of  manipulations  resulting 
in  the  distribution  of  the  coloured  layer  of  glass  in  almost 
any  desired  manner  over  the  object  in  hand.  The  principal 
objection  to  this  process,  however,  lies  in  the  fact  that  pots 
of  molten  glass  of  all  the  colours  desired  must  be  kept 
available  to  the  blower  at  the  same  time,  and  this  is  not 
easily  arranged  for  in  any  reasonably  economical  manner. 
For  this  reason,  and  also  because  the  manipulations  are 
simpler,  coloured  glass  intended  for  application  to  blown 
glass-ware  is  generally  used  in  the  form  of  short  rods 
previously  prepared ;  these  rods  are  suitably  heated,  and  the 
coloured  glass  can  then  be  applied  to  the  article  in  hand  at 
any  desired  place  and  in  as  small  or  large  a  quantity  as 
required.  If  the  two  glasses  thus  brought  into  contact  are 
properly  related  to  one  another  as  regards  chemical  composi- 
tion and  physical  properties,  they  blend  very  readily  and 
perfectly,  and  the  result  is  quite  as  good  as  could  be  obtained 
by  using  the  coloured  glass  in  the  molten  condition.  Other 
decorations,  such  as  gilding  or  other  metallic  lustres  and 
also  various  kinds  of  iridescence,  are  produced  upon  the 
finished  glass.  Metallic  lustres  are  obtained  by  placing 
upon  the  surface  of  the  glass,  and  slightly  fusing  into  it  a 
layer  of  particles  of  the  actual  metal.  In  some  cases  this  is 
done  by  rolling  the  glass  vessel,  while  still  hot,  in  a  mass 
of  metallic  foil  of  the  kind  desired,  when  a  sufficient  quantity 
readily  adheres ;  in  other  cases  the  metal  is  applied  in  the 
form  of  a  flux  or  glaze  containing  a  large  proportion  of  an 


BLOWN  AND  PKESSED  GLASS.  115 

easily-reduced  compound  of  the  metal,  and  this  is  afterwards 
reduced  to  the  metallic  state  by  the  action  of  heat,  some- 
times aided  by  that  of  smoke  or  other  reducing  gases.  An 
iridescent  surface  is  produced  upon  certain  varieties  of 
glass  by  the  corrosive  action  of  acid  vapours ;  in  fact,  in 
localities  where  the  atmosphere  is  tainted  with  sulphur 
fumes  it  is  quite  usual  to  see  an  iridescent  lustre  on  the 
surface  of  ordinary  window  glass.  There  are,  of  course, 
numerous  other  means  of  decorating  blown  and  other  glass, 
such  as  cutting,  engraving,  etching,  silvering,  etc.,  but  it  would 
lie  beyond  the  scope  of  the  present  volume  to  deal  with  these, 
since  they  are  outside  the  field  of  actual  glass  manufacture. 
In  the  production  of  hollow  glass-ware  by  hand,  the  glass- 
blower  avails  himself  to  the  full  of  the  property  so  charac- 
teristic of  glass  of  assuming  a  pasty  or  viscous  condition 
when  suitably  heated ;  by  raising  or  lowering  the  temperature 
of  his  material,  the  blower  can  at  will  render  it  stiffer  or 
more  fluid  ;  by  blowing  he  can  distend  it,  draw  it  out  by 
the  aid  of  gravity  or  centrifugal  action,  or  he  can  mould  it 
with  the  aid  of  rods  and  tongs  of  suitable  shape,  while  at 
times  he  allows  it  to  fall  or  festoon  under  its  own  weight 
while  held  aloft.  With  all  these  manipulations  at  his 
disposal,  the  skilful  operative  is  able  to  work  the  glass  to 
his  will  and  to  fashion  objects  of  great  variety  and  beauty, 
but  it  should  be  noted  that  objects  produced  by  hand  in  this 
way  will  bear  the  mark  of  the  processes  employed  in  their 
production  in  the  fact  that  they  do  not  possess  the  extreme 
regularity  of  size  and  shape  which  are  associated  with 
machine-made  articles  ;  there  is  a  certain  natural  variability 
in  the  exact  shape  of  curves  and  festoons  that  is  foreign  to 
tbe  products  of  mechanical  processes.  For  some  purposes 

i  2 


116  GLASS  MANUFACTURE. 

this  variability  is  a  disadvantage,  while  to  some  minds  it 
appears  as  a  defect,  and  methods  have  been  devised  for 
facilitating  the  production  of  strictly  uniform  glass-ware  by 
the  use  of  moulds  as  an  aid  to  the  work  of  the  glass-blower. 
While  undoubtedly  reducing  the  value  and  beauty  of  the 
ware  from  the  purely  artistic  standpoint,  these  aids  to  hand- 
work have  rendered  possible  an  immense  expansion  of  the 
entire  industry,  since,  with  the  use  of  moulds,  presentable 
glass-ware  can  be  produced  by  hands  far  less  skilled  than 
those  required  for  pure  hand-work. 

In  the  description  given  above  of  bottle- blowing  by  hand 
we  have  already  seen  an  example  of  the  use  of  moulds  in 
aiding  the  blower  to  form  his  object  to  the  desired  size  and 
shape.  Much  more  complicated  and  decorative  objects  can, 
however,  be  produced  by  the  use  of  moulds.  Such  objects  as 
globes  and  shades  for  gas,  oil  and  electric  lamps,  when  of  a 
light  substance  and  suitable  shape,  are  usually  produced  by 
blowing  bulbs  of  glass  into  moulds,  where  they  acquire  the 
general  shape  as  well  as  the  detailed  decorated  surface  con- 
figuration which  they  afterwards  present.  Here  again  the 
body  remains  a  closed  vessel,  and  is  only  opened  and 
trimmed  to  the  final  shape  at  the  end  of  the  operation 
when  all  the  blowing  and  moulding  have  been  done. 
Articles  blown  in  this  way  very  frequently  show  "mould 
marks,"  since  the  contact  of  the  hot  glass  with  the  relatively 
cold  surface  of  the  mould  results  in  a  certain  crinkling  or 
roughening  of  the  glass,  much  as  in  the  process  of  rolling. 
This  effect  can  be  minimised  by  dressing  the  interior  sur- 
faces of  the  moulds  with  suitable  greasy  dressings,  whose 
chief  property  should  be  that  they  do  not  stick  to  the  hot 
glass  and  leave  little  or  no  residue  when  gradually  burnt 


BLOWN  AND  PKESSEB   GLASS.  117 

away  in  the  mould ;  the  proper  care  of  the  moulds  and 
their  maintenance  is  in  fact  the  first  essential  to  successful 
manufacture  in  this  as  well  as  in  the  pressed-glass  industry. 
Even  under  the  most  favourable  conditions,  however,  the 
surface  of  glass  blown  into  moulds  is  not  so  good  as  that  of 
hand-blown  articles  which  have  never  come  into  contact 
with  cold  materials,  and  therefore  retain  undiminished  the 
natural  "  fire  polish  "  which  glass  possesses  when  allowed 
to  cool  freely  from  the  molten  state.  An  effort  at  pro- 
ducing a  similar  brilliance  of  surface  on  moulded  and 
pressed  articles  is  often  made  by  exposing  them,  after  they 
have  attained  their  final  form,  to  the  heat  of  a  furnace  to 
such  an  extent  as  to  soften  the  surfaces  and  allow  the  glass 
to  re-solidify  under  the  undisturbed  influence  of  surface- 
tension  much  as  it  would  do  in  solidifying  freely  in  the  first 
place.  Unfortunately  this  process  cannot  be  carried  out 
without  more  or  less  softening  the  entire  article,  so  that 
skilful  manipulation  is  required  to  prevent  serious  deforma- 
tion of  the  object,  while  a  certain  amount  of  rounding  off 
in  all  sharp  corners  and  angles  cannot  be  avoided. 

The  air-pressure  required  to  bring  the  whole  of  the 
surfaces  of  a  large  and  possibly  complicated  piece  of  glass 
into  contact  with  the  surfaces  of  the  mould  is  sometimes 
very  considerable,  and  the  lung-power  of  the  blower  is  often 
insufficient  for  the  purpose  ;  in  many  works,  therefore, 
compressed  air  is  supplied  for  the  purpose,  arrangements 
being  employed  whereby  the  operative  can  quickly  connect 
the  mouthpiece  of  his  pipe  with  the  air-main,  while  he  can 
accurately  control  the  pressure  by  means  of  a  suitable 
valve.  The  Sievert  process  of  moulding  by  the  aid  of  steam 
pressure  has  already  been  described. 


118  GLASS  MANUFACTURE. 

Although  the  evolution  of  the  industry  scarcely  followed 
this  path,  it  is  not  a  large  step  to  pass  from  a  process  in 
which  air  pressure  is  used  to  drive  viscous  glass  into 
contact  with  a  mould  to  a  process  in  which  the  pressure  of 
the  air  is  replaced  by  the  pressure  of  a  suitably-shaped 
solid  plunger,  and  this  is  essentially  the  widely-used  process 
of  glass  pressing.  In  the  first  instance  this  mode  of  manu- 
facture is  obviously  applicable  to  solid  or  flat  and  shallow 
articles  which  could  not  be  conveniently  evolved  from  the 
spherical  bulb  which  stands  as  embryo  of  all  blown  glass  ; 
at  first  sight  it  would  seem  in  fact  as  though  the  process 
must  be  limited  to  articles  of  such  a  shape  that  a  plunger 
can  readily  enter  and  leave  the  concave  portions.  By  the 
ingenious  device,  however,  of  pressing  two  halves  of  a 
closed  or  nearly  closed  vessel  simultaneously  in  two  adjacent 
moulds  and  then  pressing  the  two  halves  together  while 
still  hot  enough  to  unite,  it  has  been  made  possible  to  pro- 
duce by  the  press  alone  such  objects  as  water- jugs,  for 
example,  into  which  a  plunger  could  not  possibly  be  intro- 
duced when  finished.  The  process  of  pressing  being  a 
purely  mechanical  one  and  requiring  no  very  elaborate 
plant  and  little  skilled  labour,  has  placed  upon  the  market 
a  host  of  cheap  and  extremely  useful  articles,  thus  serving 
to  widen  very  considerably  the  useful  applications  of  glass. 
On  the  other  hand,  the  process  has  been  and  is  still  used  to 
some  extent  for  the  production  of  articles  intended  to 
imitate  the  products  of  other  processes  such  as  hand-blown 
and  cut  glass,  with  the  result  that  a  great  deal  of  glass  has 
been  produced  which  cannot  possibly  be  classed  as  beautiful 
and  much  of  which  can  lay  as  little  claim  to  utility. 

The   essential  feature    of    the  process   of    glass   press- 


BLOWN  AND   PEESSED   GLASS.  119 

ing  consists,  as  already  indicated,  in  forcing  a  layer  of 
glass  into  contact  with  a  mould  by  the  pressure  of  a 
mechanically  actuated  plunger.  For  this  purpose  a,  suit- 
able mould  and  plunger  as  well  as  a  press  for  holding  the 
former  and  actuating  the  latter  are  required.  The  moulds 
are  generally  made  of  a  special  quality  of  close-grained 
cast-iron,  and  they  are  kept  trimmed  and  dressed  in  much 
the  same  manner  as  the  moulds  used  for  blowing  (except 
that  the  latter  are  sometimes  made  of  wood).  For  the 
purpose  of  facilitating  the  removal  of  the  finished  article, 
the  moulds  are  generally  made  in  several  pieces  which  fit 
into  one  another  and  can  be  separated  by  means  of  hinges. 
A  very  important  point  about  these  moulds  is  that  the 
various  pieces  should  fit  accurately  into  one  another,  since 
otherwise  a  minute."  fin  "  of  glass  will  be  forced  into  every 
interstice,  and  the  traces  of  these  fins  will  always  remain 
visible  on  the  finished  article  ;  the  very  perfect  fit  required 
to  entirely  prevent  the  formation  of  such  fins  is,  of  course, 
scarcely  attainable  in  practice  except  in  the  case  of  new 
moulds,  so  that  the  traces  of  fins  are  generally  to  be  found 
on  all  pressed  articles,  anil  serve  as  a  ready  means  of 
identifying  these  products  when  an  attempt  is  made  to 
imitate  better  classes  of  glass-ware  by  their  means.  The 
presses  used  in  this  process  are  generally  of  the  hand-lever 
type  ;  power  presses  could  no  doubt  be  used,  but  it  is  con- 
tended that  the  hand-press  has  a  very  great  advantage  in 
allowing  the  operator  to  judge  by  touch  when  sufficient 
pressure  has  been  exerted,  and  this  is  an  important  con- 
sideration, since  an  excessive  pressure  would  either  force 
the  glass  out  of  the  mould  altogether  or  would  be  liable  to 
burst  or  injure  the  mould  seriously.  The  actual  presses 


120  GLASS  MANUFACTUEE. 

consist  of  vertical  guides  and  levers  for  controlling  the 
movement  of  the  plunger  and  a  table  for  holding  the 
moulds,  and  in  some  cases  a  system  of  cranks  and  levers 
for  opening  and  closing  the  moulds.  The  process  of 
pressing  is  exceedingly  simple.  The  proper  quantity  of 
glass  is  gathered  from  the  pot  on  a  solid  rod  and  dropped 
into  the  mould.  The  thread  of  glass  which  remains 
between  the  glass  in  the  mould  and  that  remaining  on  the 
iron  is  cut  off  with  a  pair  of  shears,  and  then  the  plunger  is 
lowered  into  the  mould  and  allowed  to  remain  there  until 
the  glass  has  stiffened  sufficiently  to  retain  its  shape,  when 
the  plunger  is  withdrawn.  In  this  proceeding  it  will  be 
seen  that  the  glass  is  forced  into  intimate  contact  with  the 
relatively  cold  surfaces  of  mould  and  plunger,  and  while 
undergoing  this  treatment  the  glass  must  remain  sufficiently 
plastic  to  readily  adapt  itself  to  the  configuration  of  the 
mould.  It  is  therefore  not  surprising  to  find  that  the 
pressing  process  can  only  be  used  successfully  with  glass  of 
a  kind  specially  adapted  for  it.  Certain  varieties  of  flint 
glass  and  some  barium  glasses  are  used  for  this  purpose, 
but  the  greater  quantity  of  pressed  glass,  particularly  as 
produced  on  the  Continent,  is  made  of  a  lime-alkali  silicate 
containing  considerable  quantities  of  both  soda  and  potash 
and  relatively  little  lime  ;  while  sufficiently  resistant  for 
most  purposes,  this  glass  is  particularly  soft  and  adaptable 
while  in  the  viscous  condition. 

The  deleterious  effect  produced  upon  glass  surfaces  when 
brought  into  contact  with  relatively  cold  metal  has  already 
been  referred  to  above,  and  it  only  remains  to  add  that  this 
is  the  principal  difficulty  with  which  the  glass-pressing 
process  has  to  contend.  It  is  overcome  to  some  extent  by 


BLOWN  AND  PEESSED   GLASS.  121 

the  aid  of  the  reheating  process  described  above ;  but  this 
is  only  a  partial  remedy,  and  in  the  majority  of  pressed 
glass  products  the  surface  is  "  covered  "  as  far  as  possible 
by  the  application  of  relief  decorations  such  as  grooves, 
spirals,  and  ribbings.  An  attempt  is  sometimes  made  to 
imitate  the  appearance  of  cut  glass,  but  the  rounding  of  the 
angles  during  the  reheating  process  destroys  the  sharpness 
of  the  effect  and  allows  of  the  ready  detection  of  the 
imitation,  while  the  cheapness  of  the  decoration  when 
applied  in  the  mould  has  frequently  led  manufacturers  to 
grossly  over-decorate,  and,  therefore,  destroy  all  claim  to 
beauty  in  their  wares. 


CHAPTER   IX. 

ROLLED    OR    PLATE-GLASS. 

IN  the  present  chapter  we  propose  to  deal  with  all  those 
processes  of  glass  manufacture  in  which  the  first  stage 
consists  in  converting  the  glass  into  a  slab  or  plate  by  some 
process  of  rolling.  We  have  already  considered  the  general 
character  of  the  rolling  process,  and  have  seen  that, 
although  hot,  viscous  glass  lends  itself  readily  to  being 
rolled  into  sheets  or  slabs,  these  cannot  be  turned  out  with 
a  smooth,  flat  surface.  In  practice  the  surface  of  rolled  glass 
is  always  more  or  less  dimmed  by  contact  with  the  minute 
irregularities  of  table  or  roller,  and  larger  irregularities 
of  the  surface  arise  from  the  buckling  that  occurs  at  a 
great  many  places  in  the  sheet.  These  limitations  govern 
the  varieties  of  glass  that  can  be  produced  by  processes 
that  involve  rolling,  and  have  led  to  the  somewhat  curious 
result  that  both  the  cheapest  and  roughest,  as  well  as  the 
best  and  most  expensive  kinds  of  flat  glass,  are  produced 
by  rolling  processes.  Ordinary  rough  "rolled  plate,"  such 
as  that  used  in  the  skylights  of  workshops  and  of  railway 
stations,  is  the  extreme  on  the  one  hand,  while  polished 
plate-glass  represents  the  other  end  of  the  scale.  The 
apparent  paradox  is,  however,  solved  when  it  is  noted  that 
in  the  production  of  polished  plate-glass  the  character  of 


ROLLED  OE  PLATE-GLASS.  123 

the  surface  of  the  glass  as  it  leaves  the  rollers  is  of  very 
minor  importance,  since  it  is  entirely  obliterated  by  the 
subsequent  processes  of  grinding,  smoothing,  and  polishing. 
Intermediate  between  the  rough  "  rolled "  and  the 
"polished"  plate-glass  we  have  a  variety  of  glasses  in 
which  the  appearance  of  the  rolled  surface  is  hidden  or 
disguised  to  a  greater  or  lesser  extent  by  the  application  of 
a  pattern  that  is  impressed  upon  the  glass  during  the 
rolling  process ;  thus  we  have  rolled  plate  having  a  ribbed 
or  lozenge-patterned  surface,  or  the  well-known  variety  of 
''figured  rolled"  plate,  sometimes  known  as  "  Muranese," 
whose  elaborate  and  deeply-imprinted  patterns  give  a  very 
brilliant  effect. 

Eolled  plate-glass  being  practically  the  roughest  and 
cheapest  form  of  glazing,  is  principally  employed  where 
appearance  is  not  considered,  and  its  chief  requirement  is, 
therefore,  cheapness,  although  both  the  colour  and  quality 
of  the  glass  are  of  importance  as  affecting  the  quantity  and 
character  of  the  light  which  it  admits  to  the  building  where 
the  glass  is  used.  On  the  ground  of  cheapness  it  will  be 
obvious  from  what  we  have  said  above  (Chapter  IV.),  that 
such  glass  can  only  be  produced  economically  in  large  tank 
furnaces,  and  these  are  universally  used  for  this  purpose. 
The  requirements  as  regards  freedom  from  enclosed  foreign 
bodies  of  small  size  and  of  enclosed  air-bells  are  not  very 
high  in  such  glass,  and,  therefore,  tanks  of  very  simple 
form  are  generally  used.  No  refinements  for  regulating 
the  temperature  of  various  parts  of  the  furnace  in  order  to 
ensure  perfect  fining  of  the  glass  are  required,  and  the 
furnace  generally  consists  simply  of  an  oblong  chamber  or 
tank,  at  one  end  of  which  the  raw  materials  are  fed  in, 


124  GLASS  MANUFACTUKE. 

while  the  glass  is  withdrawn  by  means  of  ladles  from  one 
or  two  suitable  apertures  at  the  other  end.  For  economical 
working,  however,  the  furnace  must  be  capable  of  working 
at  a  high  temperature,  because  a  cheap  glass  mixture  is 
necessarily  somewhat  infusible,  at  all  events  where  colour 
is  considered.  This  will  be  obvious  if  we  remember  that 
the  fusibility  of  a  glass  depends  upon  its  alkali  contents, 
and  alkali  is  the  most  expensive  constituent  of  such 


The  actual  raw  materials  used  in  the  production  of 
rolled  plate-glass  are  sand,  limestone  and  salt-cake,  with 
the  requisite  addition  of  carbon  and  of  fluxing  and  purifying 
materials.  The  selection  of  these  materials  is  made  with 
a  view  to  the  greatest  purity  and  constancy  of  composition 
which  is  available  within  the  strictly- set  limits  of  price 
which  the  low  value  of  the  finished  product  entails.  These 
materials  are  handled  in  very  large  quantities,  outputs  of 
from  60  to  150  tons  of  finished  glass  per  week  from  a 
single  furnace  being  by  no  means  uncommon ;  mechanical 
means  of  handling  the  raw  materials  and  of  charging 
them  into  the  furnace  are  therefore  adopted  wherever 
possible. 

*  The  glass  is  withdrawn  from  the  furnace  by  means  of 
large  iron  ladles.  These  ladles  are  used  of  varying  sizes 
in  such  a  way  as  to  contain  the  proper  amount  of  glass  to 
roll  to  the  various  sizes  of  sheets  required.  The  sizes  used 
are  sometimes  very  large,  and  ladles  holding  as  much  as 
180  to  200  Ibs.  of  glass  are  used.  These  ladles,  when  filled 
with  glass,  are  not  carried  by  hand,  but  are  suspended 
from  slings  attached  to  trolleys  that  run  on  an  overhead 
rail.  The  ladler,  whose  body  is  protected  by  a  felt  apron 


EOLLED   OE  PLATE-GLASS.  125 

and  his  face  by  a  mask  having  view-holes  glazed  with 
green  glass,  takes  the  empty  ladle  from  a  water-trough,  in 
which  it  has  been  cooled,  carries  it  to  the  slightly  inclined 
gangway  that  leads  up  to  the  opening  in  the  front  of  the 
furnace,  and  there  introduces  the  ladle  into  the  molten 
glass,  giving  it  a  half-turn  so  as  to  fill  it  with  a  "  solid  " 
mass  of  glass.  By  giving  the  ladle  two  or  three  rapid 
upward  jerks,  the  operator  then  detaches  the  glass  in  the 
ladle  as  far  as  possible  from  the  sheets  and  threads  of 
glass  which  would  otherwise  follow  its  withdrawal ;  then 
the  part  of  the  handle  of  the  ladle  near  the  bowl  is  placed 
in  the  hook  attached  to  the  overhead  trolley,  and  by  bearing 
his  weight  on  the  other  end  of  the  handle,  the  workman 
draws  the  whole  ladle  up  from  the  molten  bath  in  the 
furnace  and  out  through  the  working  aperture.  This 
operation  only  takes  a  few  seconds  to  perform,  but  during 
this  time  the  ladler  is  exposed  to  great  heat,  as  a  more  or 
less  intense  flame  generally  issues  from  the  working  aper- 
ture, whence  it  is  drawn  upward  under  the  hood  of  the 
furnace.  From  the  furnace  opening,  the  ladler,  generally 
aided  by  a  boy,  runs  the  full  ladle  to  the  rolling  table  and 
there  empties  the  ladle  upon  the  table  just  in  front  of  the 
roller.  In  doing  this,  two  distinctly  different  methods  are 
employed.  In  one,  only  the  perfectly  fluid  portion  of  the 
glass  is  poured  out  of  the  ladle  by  gradually  tilting  it,  the 
chilled  glass  next  to  the  walls  of  the  ladle  being  retained 
there  and  ultimately  returned  to  the  furnace  while  still  hot. 
In  the  other  method,  the  chilling  of  the  glass  is  minimised 
as  far  as  possible,  and  the  entire  contents  of  the  ladle  are 
emptied  upon  the  rolling  table  by  the  ladler,  who  turns  the 
entire  ladle  over  with  a  rapid  jerk  which  is  so  arranged 


126 


GLASS  MANUFACTUEE. 


as  to  throw  the  coldest  part  of  the  glass  well  away  from  the 
rest.  When  the  sheet  is  subsequently  rolled  this  chilled 
portion  is  readily  recognised  by  its  darker  colour,  and  since 
it  lies  entirely  at  one  end  of  the  sheet  it  is  detached  before 
the  sheet  goes  any  further.  Neither  method  appears  to 
present  any  preponderating  advantage. 

The  rolling  table  used  in  the  manufacture  of  rolled  plate 
is  essentially  a  cast-iron  slab  of  sufficient  size  to  accommo- 


GUIDES 


VlM 

Q  O 

SECTIONAL   ELEVATION. 


GUIDES 

TABLE 
MOLTEN  GLASS 


DIRECTION  OF 
ROLLING 


PLAN. 

FIG.  9. — Boiling  table  for  rolled  plate-glass. 

date  the  largest  sheet  which  is  to  be  rolled ;  over  this  slab 
moves  a  massive  iron  roller  which  may  be  actuated  either 
by  hand  or  by  mechanical  power — the  latter,  however, 
being  now  almost  universal.  The  thickness  of  the  sheet  to 
be  rolled  is  regulated  by  means  of  slips  of  iron  placed  at 
the  sides  of  the  table  in  such  a  way  as  to  prevent  the 
roller  from  descending  any  further  towards  the  surface  of 
the  table :  so  long  as  the  layer  of  glass  is  thicker  than 
these  slips,  the  entire  weight  of  the  roller  comes  upon  the 


EOLLED  OE   PLATE-GLASS.  127 

soft  glass  and  presses  it  down,  but  as  soon  as  the  required 
thickness  is  attained,  the  weight  of  the  roller  is  taken  by 
the  iron  slips  and  the  glass  is  not  further  reduced  in  thick- 
ness. The  width  of  the  sheet  is  regulated  by  means  of  a 
pair  of  iron  guides,  formed  to  fit  the  forward  face  of  the 
roller  and  the  surface  of  the  table,  in  the  manner  indicated 
in  Fig.  9.  The  roller,  as  it  moves  forward,  pushes  these 
guides  before  it,  and  the  glass  is  confined  between  them. 
When  the  roller  has  passed  over  the  glass,  the  sheet  is  left 
on  the  iron  table  in  a  red-hot,  soft  condition,  and  it  must 
be  allowed  to  cool  and  harden  to  a  certain  extent  before  it 
can  be  safely  moved.  In  this  interval,  the  chilled  portion 
—if  any — is  partially  severed  by  an  incision  made  in  the 
sheet  by  means  of  a  long  iron  implement  somewhat  like 
a  large  knife,  and  then  the  sheet  is  loosened  from  the  bed 
of  the  table  by  passing  under  it,  with  a  smooth  rapid  stroke, 
a  flat-bladed  iron  tool.  The  sheet  is  next  removed  to 
the  annealing  kiln  or  "  lear,"  being  first  drawn  on  to  a  stone 
slab  and  thence  pushed  into  the  mouth  of  the  kiln.  At 
this  stage  the  chilled  portion  of  the  sheet  is  completely 
severed  by  a  blow  which  causes  the  glass  to  break  along  the 
incision  previously  made. 

The  rolled-plate  annealing  kiln  is  essentially  a  long,  low 
tunnel,  kept  hot  at  one  end,  where  the  freshly-rolled  sheets 
are  introduced,  and  cold  at  the  other  end,  the  temperature 
decreasing  uniformly  down  the  length  of  the  tunnel.  The 
sheets  pass  down  this  tunnel  at  a  slow  rate,  and  are  thus 
gradually  cooled  and  annealed  sufficiently  to  undergo  the 
necessary  operations  of  cutting,  etc.  Although  thus  simple 
in  principle,  the  proper  design  and  working  of  these  "lears  " 
is  by  no  means  simple  or  easy,  since  success  depends  upon  the 


128  GLASS  MANUFACTUEE. 

correct  adjustment  of  temperatures  throughout  the  length 
of  the  tunnel  and  a  proper  rate  of  movement  of  the  sheets, 
while  the  manner  of  handling  and  supporting  the  sheets 
is  vital  to  their  remaining  flat  and  unbroken.  The  actual 
movement  of  the  sheets  is  effected  by  a  system  of  moving 
grids  which  run  longitudinally  down  the  tunnel.  The 
sheets  ordinarily  lie  flat  upon  the  stone  slabs  that  form  the 
floor  of  the  tunnel,  and  the  grids  are  lowered  into 
recesses  cut  to  receive  them.  At  regular  intervals  the  iron 
grid  bars  are  raised  just  sufficiently  to  lift  the  sheets  from 
the  bed  of  the  kiln,  and  are  then  moved  longitudinally  a 
short  distance,  carrying  the  sheets  forward  with  them  and 
immediately  afterwards  again  depositing  them  on  the  stone 
bed.  The  grids  return  to  their  former  position  while 
lowered  into  their  recesses  below  the  level  of  the  kiln  bed. 

When  they  emerge  from  the  annealing  kiln  or  "  lear  " 
the  sheets  of  rolled  plate-glass  are  carried  to  the  cutting 
and  sorting  room.  Here  the  sheets  are  trimmed  and  cut 
to  size.  The  edges  of  the  sheets  as  they  leave  the  rolling 
table  are  somewhat  irregular,  and  sometimes  a  little 
"  beaded,"  while  the  ends  are  always  very  irregular.  Ends 
and  edges  are  therefore  cut  square  or  "  trimmed  "  by  the 
aid  of  the  cutting  diamond.  For  this  purpose  the  sheet  is 
laid  upon  a  flat  table,  the  smoothest  side  of  the  sheet  being 
placed  upwards,  and  long  cuts  are  taken  with  a  diamond — 
good  diamonds  of  adequate  size  and  skilful  operators  being 
necessary  to  ensure  good  cutting  on  such  thick  glass  over 
long  lengths.  Strips  of  glass  six  or  eight  feet  long  and 
half  an  inch  wide  are  frequently  detached  in  the  course  of 
this  operation,  and  the  final  separation  is  aided  by  slight 
tapping  of  the  underside  of  the  glass  just  below  the  cut 


EOLLED   OK  PLATE-GLASS.  129 

and — if  necessary — by  breaking  the  strip  off  by  the  aid  of 
suitable  tongs. 

No  very  elaborate  "  sorting "  of  rolled  plate  glass  is 
required,  except  perhaps  that  the  shade  of  colour  in  the 
glass  may  vary  slightly  from  time  to  time,  and  it  is 
generally  preferable  to  keep  to  one  shade  of  glass  in  filling 
any  particular  order.  Apart  from  this,  the  rolled  plate 
cutter  has  merely  to  cut  out  gross  defects  which  would 
interfere  too  seriously  with  the  usefulness  of  the  glass. 
As  we  have  already  indicated,  air-bells  and  minute 
enclosures  of  opaque  matter  are  not  objectionable  in 
this  kind  of  glass,  but  large  pieces  of  opaque  material 
must  generally  be  cut  out  and  rejected,  not  only  because 
they  are  too  unsightly  to  pass  even  for  rough  glazing 
purposes,  but  also  because  they  entail  a  considerable 
risk  of  spontaneous  cracking  of  the  glass — in  fact,  visible 
cracks  are  nearly  always  seen  around  large  "stones,"  as 
these  inclusions  are  called.  These  may  arise  from  various 
causes,  such  as  incomplete  melting  of  the  raw  materials, 
or  the  contamination  of  the  raw  materials  with  infusible 
impurities,  but  the  most  fruitful  source  of  trouble  in  this 
direction  lies  in  the  crumbling  of  the  furnace  lining,  which 
introduces  small  lumps  of  partially  melted  fire-clay  into  the 
glass.  In  a  rolled  plate  tank  furnace  which  is  properly  con- 
structed and  worked,  the  percentage  of  sheets  which  have  to 
be  cut  up  on  account  of  such  enclosures  should  be  very 
small,  at  all  events  until  the  furnace  is  old,  when  the  linings 
naturally  show  an  increasing  tendency  to  disintegrate. 

Keturning  now  to  the  rolling  process,  it  is  readily  seen 
that  a  very  slight  modification  will  result  in  the  production 
of  rolled  plate-glass  having  a  pattern  impressed  upon  one 

G.M.  K 


130  GLASS  MANUEACTUKE. 

surface ;  this  modification  consists  in  engraving  upon  the 
cast-iron  plate  of  the  rolling  table  in  intaglio  any  pattern 
that  is  to  appear  upon  the  glass  in  relief.  As  a  matter  of 
fact  only  very  simple  patterns  are  produced  in  this  way, 
such  as  close  parallel  longitudinal  ribbing  and  a  lozenge- 
pattern,  the  reason  probably  being  that  the  cost  of  cutting 
an  elaborate  pattern  over  the  large  area  of  the  bed-plate  of 


FIG.  10. — Sectional  diagram  of  machine  for  rolling  "figured  rolled" 

plate-glass. 

one  of  these  tables  would  be  very  considerable.  Further, 
as  these  tables  and  their  bed-plates  are  so  very  heavy,  they 
are  not  readily  interchanged  or  left  standing  idle,  so  that 
only  patterns  required  in  very  great  quantity  could  be 
profitably  produced  in  this  way.  These  disadvantages  are, 
however,  largely  overcome  by  the  double-rolling  machine. 
In  this  machine,  into  whose  rather  elaborate  details  we 
cannot  enter  here,  the  glass  is  rolled  out  into  a  sheet  of 
the  desired  size  and  thickness  by  being  passed  between 
two  rollers  revolving  about  stationary  axes,  the  finished 
sheet  emerging  over  another  roller,  and  passing  on 


EOLLED   OE  PLATE-GLASS.  131 

to  a  stone  slab  that  moves  forward  at  the  same  rate 
as  the  sheet  is  fed  down  upon  it.  In  this  machine  a 
pattern  can  be  readily  imprinted  upon  the  soft  sheet  as 
it  passes  over  the  last  roller  by  means  of  a  fourth  roller, 
upon  which  the  pattern  is  engraved  ;  this  is  pressed  down 
upon  the  sheet,  and  leaves  upon  it  a  clear,  sharp  and  deep 
impress  of  its  pattern.  The  general  arrangement  of  the 
rollers  in  this  machine  is  shown  in  the  diagram  of  Fig.  10, 
which  represents  the  sectional  elevation  of  the  appliance. 
After  leaving  the  rolling  machine,  the  course  of  the 
"  figured  rolled  plate  "  produced  in  this  manner  is  exactly 
similar  to  that  of  ordinary  rolled  plate,  except  that  as  a 
somewhat  softer  kind  of  glass  is  generally  used  for  "figured," 
the  temperature  of  the  annealing  kilns  requires  somewhat 
different  adjustment.  The  cutting  of  the  glass  also  requires 
rather  more  care,  and  it  should  be  noted  that  such  glass 
can  only  be  cut  with  a  diamond  on  the  smooth  side  ;  the 
side  upon  which  the  pattern  has  been  impressed  in  relief 
cannot  be  materially  affected  by  a  diamond.  This  is  one 
reason  why  it  is  not  feasible  to  produce  such  glass  with  a 
pattern  on  both  sides. 

Figured  rolled  glass,  being  essentially  of  an  ornamental 
or  decorative  nature,  is  generally  produced  in  either 
brilliantly  white  glass  or  in  special  tints  and  colours,  and 
the  mixtures  used  for  attaining  these  are,  of  course,  the 
trade  property  of  the  various  manufacturers  ;  the  whiteness 
of  the  glass,  however,  is  only  obtainable  by  the  use  of  very 
pure  and,  therefore,  expensive  materials.  As  regards  the 
coloured  plate-glasses,  a  general  account  of  the  principles 
underlying  the  production  of  coloured  glass  will  be  found 
in  Chapter  XI. 

K  2 


132  GLASS   MANUFACTUEE. 

The  manufacture  of  polished  plate-glass  really  stands 
somewhat  by  itself,  almost  the  only  feature  which  it  has 
in  common  with  the  branches  of  manufacture  just  described 
being  the  initial  rolling  process. 

The  raw  materials  for  the  production  of  plate-glass  are 
chosen  with  the  greatest  possible  care  to  ensure  purity  and 
regularity ;  owing  to  the  very  considerable  thickness  of 
glass  which  is  sometimes  employed  in  plate,  and  also  to 
the  linear  dimensions  of  the  sheets  which  allow  of  numerous 
internal  reflections,  the  colour  of  the  glass  would  become 
unpleasantly  obtrusive  if  the  shade  were  at  all  pronounced. 
The  actual  raw  materials  used  vary  somewhat  from  one 
works  to  another ;  but,  as  a  rule,  -they  consist  of  sand, 
limestone,  and  salt-cake,  with  some  soda-ash  and  the  usual 
additions  of  fluxing  and  purifying  material  such  as  arsenic, 
manganese,  etc.  The  glass  is  generally  melted  in  pots, 
and  extreme  care  is  required  to  ensure  perfect  melting  and 
fining,  since  very  minute  defects  are  readily  visible  in  this 
glass  when  finished,  and,  of  course,  detract  most  seriously 
from  its  value. 

The  method  of  transferring  the  glass  from  the  melting- 
pot  to  the  rolling  table  differs  somewhat  in  different  works. 
In  many  cases  the  melting-pots  themselves  are  taken  bodily 
from  the  furnace  and  emptied  upon  the  bed-plate  of  the 
rolling  machine,  while  in  other  cases  the  glass  is  first 
transferred  to  smaller  "  casting"  pots,  where  it  has  to  be 
heated  again  until  it  has  freed  itself  from  the  bubbles 
enclosed  during  the  transference,  and  then  these  smaller 
pots  are  used  for  pouring  the  glass  upon  the  rolling  slab. 
The  advantage  of  the  latter  more  complicated  method  lies, 
no  doubt,  in  the  fact  that  the  large  melting-pots,  which 


EOLLEI)   OE  PLATE-GLASS.  133 

have  to  bear  the  brunt  of  the  heat  and  chemical  action 
during  the  early  stages  of  melting,  are  not  exposed  to  the 
great  additional  strain  of  being  taken  from  the  hot  furnace 
and  exposed  for  some  time  to  the  cold  outside  air.  Apart 
from  the  mechanical  risks  of  fracture,  this  treatment 
exposes  the  pots  to  grave  risks  of  breakage  from  unequal 
expansion  and  contraction  on  account  of  the  great  differences 
of  temperature  involved.  Where  smaller  special  casting- 
pots  are  used,  these  are  not  exposed  to  such  prolonged 
heat  in  the  furnace,  and  are  never  exposed  to  the  chemical 
action  of  the  raw  materials,  so  that  these  subsidiary  pots 
may  perhaps  be  made  of  a  material  better  adapted  to  with- 
stand sudden  changes  of  temperature  than  the  high-class 
fire-clay  which  must  be  used  in  the  construction  of  melting 
pots.  On  the  other  hand,  the  transference  of  the  glass 
from  the  melting  to  the  casting-pots  involves  a  laborious 
operation  of  ladling  and  the  refining  of  the  glass,  with  its 
attendant  expenditure  of  time  and  fuel.  Finally,  the 
production  of  plate-glass  in  tank  furnaces  could  only  be 
attempted  by  the  aid  of  such  casting-pots  in  which  the 
glass  would  have  to  undergo  a  second  fining  after  being 
ladled  from  the  tank,  and  this  would  materially  lessen  the 
economy  of  the  tank  for  this  purpose,  while  it  is  by  no 
means  an  easy  matter  to  produce  in  tank  furnaces  qualities 
of  glass  equal  as  regards  colour  and  purity  to  the  best 
products  of  the  pot-furnace. 

The  withdrawal  of  the  pots  containing  the  molten  glass 
from  the  furnace  is  now  universally  carried  out  by  powerful 
machinery.  The  pots  are  provided  on  their  outer  surface 
with  projections  by  which  they  can  be  held  in  suitably- 
shaped  tongs  or  cradles.  A  part  of  the  furnace  wall,  which 


134  GLASS  MANUFACTUEE. 

is  constructed  each  time  in  a  temporary  manner,  is  broken 
down;  the  pot  is  raised  from  the  bed  or  "siege"  of  the 
furnace  by  the  aid  of  levers,  and  is  then  bodily  lifted  out 
by  means  of  a  powerful  fork.  The  pot  is  then  lifted  and 
carried  by  means  of  cranes  until  it  is  in  position  above  the 
rolling  table ;  there  the  pot  is  tilted  and  the  glass  poured 
out  in  a  steady  stream  upon  the  table,  care  being  taken  to 
avoid  the  inclusion  of  air-bells  in  the  mass  during  the 
process  of  pouring.  When  empty,  the  pot  is  returned  to 
the  furnace  as  rapidly  as  possible,  the  glass  being  mean- 
while rolled  out  into  a  slab  by  the  machine.  Except  for 
the  greater  size  and  weight  of  both  table  and  roller,  the 
plate-glass  rolling  table  is  similar  to  that  already  described 
in  connection  with  rolled  plate.  Of  course,  since  the  glass 
is  poured  direct  from  the  pot,  there  is  no  chilled  glass  to 
be  removed.  Further,  owing  to  the  large  size  of  sheets 
frequently  required,  the  bed  of  the  rolling  table  cannot  be 
made  of  a  single  slab  of  cast-iron,  a  number  of  carefully 
jointed  plates  being,  in  fact,  preferable,  as  they  are  less 
liable  to  warp  under  the  action  of  the  hot  glass. 

In  arranging  the  whole  of  the  rolling  plant,  the  chief 
consideration  to  be  kept  in  mind  is  that  it  is  necessary  to 
produce  a  flat  sheet  of  glass  of  as  nearly  as  possible  equal 
thickness  all  over.  The  final  thickness  of  the  whole  slab 
when  ground  and  polished  into  a  sheet  of  plate-glass  must 
necessarily  be  slightly  less  than  that  of  the  thinnest  part 
of  the  rough  rolled  sheet.  If,  therefore,  there  are  any 
considerable  variations  of  thickness,  the  result  will  be  that 
in  some  parts  of  the  sheet  a  considerable  thickness  of  glass 
will  have  to  be  removed  during  the  grinding  process.  This 
will  arise  to  a  still  more  serious  extent  if  the  sheet  as  a 


EOLLED   OE  PLATE-GLASS.  135 

whole  should  be  bent  or  warped  so  as  to  depart  materially 
from  flatness.  The  two  cases  are  illustrated  diagram- 
matically  in  Fig.  11,  which  shows  sectional  views  of  the 
sheets  before  and  after  grinding  on  an  exaggerated  scale. 

While  it  is  evident  that  careful  design  of  the  rolling  table 
will  avoid  all  tendency  to  the  formation  of  sheets  of  such 
undesirable  form,  it  is  a  much  more  difficult  matter  to 
avoid  all  distortion  of  the  sheet  during  the  annealing  pro- 


FINIS  HE  D       PLATE 

GLASS  GROUND    AWAY 
PLATE    CAST   WITH    IRREGULAR    SURFACE. 

WASTE  GLASS 
FINISHED      PLATE 


WASTE  GLASS 
WARPED    OR    CURVED    PLATE 

FIG.  11. — Sectional  diagram  illustrating  waste  of  glass  in  grinding 
curved  or  irregular  plate. 

cess  and  while  the  sheet  is  being  moved  from  the  rolling- 
table  to  the  annealing  kiln.  Owing  to  the  great  size  of  the 
slabs  of  glass  to  be  dealt  with,  and  still  more  to  the 
stringent  requirement  of  flatness,  the  continuous  annealing 
kiln,  in  which  the  glass  travels  slowly  down  a  tunnel  from 
the  hot  to  the  cold  end,  has  not  been  adopted  for  the 
annealing  of  plate-glass,  and  a  form  of  annealing  kiln  is 
still  used  for  that  glass  which  is  similar  in  its  mode  of 
operation  to  the  old-fashioned  kilns  that  were  used  for  other 


136  GLASS  MANUFACTUBE. 

kinds  of  glass  before  the  continuous  kiln  was  introduced. 
These  kilns  simply  consist  of  chambers  in  which  the  hot 
glass  is  sealed  up  and  allowed  to  cool  slowly  and  uniformly 
during  a  more  or  less  protracted  period.  In  the  case  of 
plate-glass,  the  slabs  are  laid  flat  on  the  stone  bed  of  the 
kiln.  This  stone  bed  is  built  up  of  carefully  dressed  stone, 
or  blocks  of  fire-brick  bedded  in  sand  in  such  a  way  that  they 
can  expand  freely  laterally  without  causing  any  tendency  for 
the  floor  to  buckle  upwards  as  it  would  do  if  the  blocks  were 
set  firmly  against  one  another.  The  whole  chamber  is 
previously  heated  to  the  requisite  temperature  at  which  the 
glass  still  shows  a  very  slight  plasticity.  The  hot  glass 
slabs  from  the  rolling  table  are  laid  upon  the  bed  of  this 
kiln,  several  being  usually  placed  side  by  side  in  the  one 
chamber,  and  the  slabs  in  the  course  of  the  first  few  hours 
settle  down  to  the  contour  of  the  bed  of  the  kiln,  from 
which  shape  and  position  they  are  never  disturbed  until 
they  are  removed  when  quite  cold.  In  modern  practice 
the  cooling  of  a  kiln  is  allowed  to  occupy  from  four  to  five 
days ;  even  this  rate  of  cooling  is  only  permissible  if  care 
is  taken  to  provide  for  the  even  cooling  of  all  parts  of  the 
kiln,  and  for  this  purpose  special  air-passages  are  built 
into  the  walls  of  the  chamber  and  beneath  the  bed  upon 
which  the  glass  rests,  and  air  circulation  is  admitted  to 
these  in  such  a  way  as  to  allow  the  whole  of  the  kiln  to  cool 
down  at  the  same  rate  ;  in  the  absence  of  such  special 
arrangements,  the  upper  parts  of  the  kiln  would  probably 
cool  much  more  rapidly  than  the  base,  so  that  the  glass 
would  be  much  warmer  on  its  under  than  on  its  upper  surface. 
When  the  slabs  of  plate-glass  are  removed  from  the 
annealing  kilns  they  very  closely  resemble  sheets  of  rolled 


EOLLED   OE  PLATE-GLASS.  137 

plate  in  appearance,  and  they  are  quite  sufficiently  trans- 
parent to  allow  of  examination  and  the  rejection  of  the 
more  grossly  defective  portions  ;  the  more  minute  defects, 
of  course,  can  only  be  detected  after  the  sheets  have  been 
polished,  but  this  preliminary  examination  saves  the 
laborious  polishing  of  much  useless  glass. 

The  process  of  grinding  and  polishing  plate-glass  consists 
of  three  principal  stages.  In  the  first  stage  the  surfaces  of 
the  glass  are  ground  so  as  to  be  as  perfectly  flat  and  parallel 
as  possible ;  in  order  to  effect  this  object  as  rapidly  as  possible, 
a  coarse  abrasive  is  used  which  leaves  the  glass  with  a 
rough  grey  surface.  In  the  second  stage,  that  of  smoothing, 
these  rough  grey  surfaces  are  ground  down  with  several 
grades  of  successively  finer  abrasive  until  finally  an 
exceedingly  smooth  grey  surface  is  left.  In  the  third  and 
final  stage,  the  smooth  grey  surface  is  converted  into  the 
brilliant  polished  surface  with  which  we  are  familiar  by 
the  action  of  a  polishing  medium. 

Originally  the  various  stages  of  the  grinding  and  polish- 
ing processes  were  carried  out  by  hand,  but  a  whole  series 
of  ingenious  machines  has  been  produced  for  effecting  the 
same  purpose  more  rapidly  and  more  perfectly  than  hand- 
labour  could  ever  do.  We  cannot  hope  to  give  any 
detailed  account  of  the  various  systems  of  grinding  and 
polishing  machines  which  are  even  novv  in  use,  but  must 
content  ourselves  with  a  survey  of  some  of  the  more  impor- 
tant considerations  governing  the  design  and  construction 
of  such  machinery. 

In  the  first  place,  before  vigorous  mechanical  work  can 
be  applied  to  the  surface  of  a  plate  of  glass,  that  plate  must 
be  firmly  fixed  in  a  definite  position  relatively  to  the  rest 


138  GLASS  MANUFACTUKE. 

of  the  machinery,  and  such  firm  fixing  of  a  plate  of  glass  is 
by  no  means  readily  attained,  since  the  plate  must  be 
supported  over  its  whole  area  if  local  fracture  is  to  be 
avoided.  While  the  surface  of  the  plate  is  in  the  uneven 
condition  in  which  it  leaves  the  rolling-table,  such  a  firm 
setting  of  the  glass  can  only  be  attained  by  bedding  it  in 
plaster,  and  this  must  be  done  in  such  a  manner  as  to 
avoid  .the  formation  of  air-bubbles  between  plaster  and 
glass  ;  if  bubbles  are  allowed  to  form,  they  constitute  places 
where  the  glass  is  unsupported.  During  the  grinding  and 
polishing  processes  these  unsupported  places  yield  to  the 
heavy  pressure  that  comes  upon  them,  and  irregularities  in 
the  finished  polished  surfaces  result.  The  most  perfect 
adhesion  between  glass  and  plaster  is  attained  by  spreading 
the  paste  of  plaster  on  the  up-turned  surface  of  the  slab  of 
glass  and  lowering  the  iron  bed-plate  of  the  grinding  table 
down  upon  it,  the  bed-plate  with  the  adhering  slab  of  glass 
being  afterwards  turned  over  and  brought  into  position  in 
the  grinding  machine.  When  one  side  of  the  glass  has 
been  polished,  it  is  generally  found  sufficient  to  lay  the 
slab  down  on  a  bed  of  damp  cloth,  to  which  it  adheres  very 
firmly,  although  sliding  is  entirely  prevented  by  a  few 
blocks  fixed  to  the  table  in  such  a  way  as  to  abut  against 
the  edges  of  the  sheet.  In  many  works,  however,  the  glass 
is  set  in  plaster  for  the  grinding  and  polishing  of  the 
second  side  as  well  as  of  the  first. 

The  process  of  grinding  and  polishing  is  still  regarded 
in  many  plate-glass  works  as  consisting  of  three  distinct 
processes,  known  as  rough  grinding,  smoothing  and  polish- 
ing respectively.  Formerly  these  three  stages  of  the  process 
were  carried  out  separately ;  at  first  by  hand,  and  later  by 


KOLLED   OR  PLATE-GLASS.  139 

three  different  machines.  In  the  most  modern  practice, 
however,  the  rough  and  smooth  grinding  are  done  on  the 
same  machine,  the  only  change  required  being  the  substi- 
tution of  a  finer  grade  of  abrasive  at  each  step  for  the 
coarser  grade  used  in  the  previous  stage.  For  the  polish- 
ing process,  however,  the  rubbing  implements  themselves 
must  be  of  a  different  kind,  for  while  the  grinding  and 
smoothing  is  generally  done  by  means  of  cast-iron  rubbers 
moving  over  the  glass,  the  polishing  is  done  with  felt  pads. 
The  table  of  the  machine,  to  which  the  glass  under  treat- 
ment is  attached,  is  therefore  made  movable,  and  when  the 
grinding  and  smoothing  processes  are  complete,  the  table 
with  its  attached  glass  is  moved  so  as  to  come  beneath  a 
superstructure  carrying  the  polishing  rubbers,  and  the 
whole  is  then  elevated  so  as  to  allow  the  rubbers  to  bear  on 
the  glass. 

The  earliest  forms  of  grinding  machines  gave  a  reciprocal 
motion  to  the  table  which  carries  the  glass,  or  the  grinding 
rubbers  were  moved  backward  and  forward  over  the 
stationary  table.  Kotary  machines,  however,  were  intro- 
duced and  rapidly  asserted  their  superiority,  until,  at  the 
present  time,  practically  all  plate-glass  is  ground  on  rotating 
tables,  some  of  these  attaining  a  diameter  of  over  30  ft. 
The  grinding  "rubbers  "  consist  of  heavy  iron  slabs,  or  of 
wood  boxes  shod  with  iron,  but  of  much  smaller  diameter 
than  the  grinding  table.  The  rubbers  themselves  are 
rotary,  being  caused  to  rotate  either  by  the  frictional  drive 
of  the  rotating  table  below  them,  or  by  the  action  of  inde- 
pendent driving  mechanism,  but  the  design  of  the  motions 
must  be  so  arranged  that  the  relative  motion  of  rubber  and 
glass  shall  be  approximately  the  same  at  all  parts  of  the 


140  GLASS  MANUFACTIJEE. 

glass  sheets,  otherwise  curved  instead  of  plane  surfaces 
would  be  formed.  This  condition  can  be  met  by  placing 
the  axes  of  the  rubbers  at  suitable  points  on  the  diameter 
of  the  table.  The  abrasive  is  fed  on  to  the  glass  in  the 
form  of  a  thin  paste,  and  when  each  grade  or  "  course  " 
has  done  the  work  required  of  it,  the  whole  table  is  washed 
down  thoroughly  with  water  and  then  the  next  finer  grade 
is  applied.  The  function  of  the  first  or  coarsest  grade  is 
simply  to  remove  the  surface  irregularities  and  to  form  a 
rough  but  plane  surface.  The  abrasive  ordinarily  employed 
is  sharp  sand,  but  only  comparatively  light  pressure  can 
be  applied,  especially  at  the  beginning  of  this  stage,  since 
at  that  period  the  weight  of  the  rubber  is  at  times  borne  by 
relatively  small  areas  of  glass  that  project  here  and  there 
above  the  general  level  of  the  slab.  As  these  are  ground 
away,  the  rubbers  take  a  larger  and  more  uniform  bearing, 
and  greater  pressure  can  be  applied.  The  subsequent 
courses  of  finer  abrasives  are  only  required  to  remove  the 
coarse  pittings  left  in  the  surface  by  the  action  of  the  first 
rough  grinding  sand  ;  the  finer  abrasive  replaces  the  deep 
pits  of  the  former  grade  by  shallower  pits,  and  this  is 
carried  on  in  a  number  of  steps  until  a  very  smooth 
'•'grey"  surface  is  attained  and  the  smoothing  process  is 
complete.  The  revolving  table  or  "  platform "  is  now 
detached  from  the  driving  mechanism,  and  moved  along 
suitably  placed  rails  on  wheels  provided  for  that  purpose, 
until  it  stands  below  the  polishing  mechanism.  Here  it  is 
attached  to  a  fresh  driving  mechanism,  and  it  is  then  either 
raised  so  as  to  bring  the  glass  into  contact  with  the  felt- 
covered  polishing  rubbers,  or  the  latter  are  lowered 
down  upon  the  glass.  The  polishing  rubbers  are  large 


EOLLED   OE  PLATE-GLASS.  141 

felt-covered  slabs  of  wood  or  iron  which  are  pressed  against 
the  glass  with  considerable  force ;  their  movement  is  very 
similar  to  that  of  the  grinding  rubbers,  but  in  place  of  an 
abrasive  they  are  supplied  with  a  thin  paste  of  rouge  and 
water.      The    time    required    for    the    polishing    process 
depends  upon  the  perfection  of  the  smoothing   that   has 
been  attained  ;  in  favourable  cases  two  or  three  hours  are 
sufficient  to  convert  the  "  grey  "  surface  into  a  perfectly 
polished  one ;  where,  however,  somewhat  deeper  pits  have 
been  left  in  the  glass,  the  time  required  for  polishing  may 
be  much  longer,  and  the  polish  attained  will  not  be  so 
perfect.    The  mode  of  action  of  a  polishing  medium  such  as 
rouge  is  now  recognised  to  be  totally  different  in  character 
from  that  of  even  the  finest  abrasive  ;  the  grains  of  the 
abrasive  act  by  their  hardness  and  the  sharpness  of  their 
edges,  chipping  away  tiny  particles  of  the  glass,  so  that  the 
glass  steadily  loses  weight  during  the  grinding  and  smooth- 
ing  processes.     During   the   polishing   process,   however, 
there  is  little  or  no  further  loss  of  weight,  the  glass  forming 
the  hills  or  highest  parts  of  the  minutely  pitted  surface 
being  dragged  or  smeared  over  the  surface  in  such  a  way 
as  to  gradually  fill  up  the  pits  and  hollows.     The   part 
played  by  the  polishing  medium  is  probably  partly  chemical 
and   partly   physical,    but   it    results,    together   with    the 
pressure  of  the  rubber,  in  giving  to  the  surface  molecules 
of   the  glass  a  certain  amount  of  freedom  of  movement, 
similar   to  that  of   the  molecules  of  a  viscid  liquid;  the 
surface  layers  of  glass  are  thus  enabled  to  "  flow  "  under 
the  action  of  the  polisher  and  to  smooth  out  the  surface  to 
the  beautiful  level  smoothness  which  is  so  characteristic  of 
the  surfaces  of  liquids  at  rest.     This  explanation  of  the 


142  GLASS  MANUFACTURE. 

polishing  process  enables  us  to  understand  why  the  proper 
consistency  of  the  polishing  paste,  as  well  as  the  proper 
adjustment  of  the  speed  and  pressure  of  the  rubbers,  plays 
such  an  important  part  in  successful  polishing ;  it  also 
serves  to  explain  the  well-known  fact  that  rapid  polishing 
only  takes  place  when  the  glass  surface  has  begun  to  be 
perceptibly  heated  by  the  friction  spent  upon  it. 

It  has  been  estimated  that,  on  the  average,  slabs  of  plate- 
glass  lose  one-third  of  their  original  weight  in  the  grinding 
and  polishing  processes,  and  it  is  obvious  that  the  erosion 
of  this  great  weight  of  glass  must  absorb  a  great  amount  of 
mechanical  energy,  while  the  cost  of  the  plant  and  upkeep  is 
proportionately  great.  Every  factor  that  tends  to  dimmish 
either  the  total  weight  of  glass  to  be  removed  per  square  yard 
of  finished  plate,  or  reduces  the  cost  of  removal,  must  be  of  the 
utmost  importance  in  this  manufacture.  The  flatness  of  the 
plates  as  they  leave  the  annealing  kiln  has  already  been 
referred  to,  and  the  reason  why  the  processes  of  grinding  and 
polishing  have  formed  the  subject  for  innumerable  patents 
will  now  be  apparent.  The  very  large  expansion  of  the  use 
of  plate-glass  in  modern  building  construction,  together 
with  the  steady  reduction  in  the  prices  of  plate,  are  evidence 
of  the  success  that  has  attended  the  efforts  of  inventors 
and  manufacturers  in  this  direction. 

At  the  present  time,  plate-glass  is  manufactured  in  very 
large  sheets,  measuring  up  to  26  ft.  in  length  by  14  ft. 
in  width,  and  in  thickness  varying  from  T%th  of  an  inch 
up  to  1J  in.,  or  more,  for  special  purposes.  At  the 
same  time  the  quality  of  the  glass  is  far  higher  to-day  than 
it  was  at  earlier  times.  This  high  quality  chiefly  results 
from  more  careful  choice  of  raw  materials  and  greater 


EOLLED  OE  PLATE-GLASS.  143 

freedom  from  the  defects  arising  during  the  melting  and 
refining  processes,  while  a  rigid  process  of  inspection  is 
applied  to  the  glass  as  it  comes  from  the  polishing  machines. 
For  this  purpose  the  sheets  are  examined  in  a  darkened  room 
by  the  aid  of  a  lamp  placed  in  such  a  way  that  its  oblique 
rays  reveal  every  minute  imperfection  of  the  glass ;  these 
imperfections  are  marked  with  chalk,  and  the  plate  is 
subsequently  cut  up  so  as  to  avoid  the  defects  that  have 
thus  been  detected. 

Perhaps  the  most  remarkable  fact  about  the  quality  of 
modern  plate-glass  is  its  relatively  high  degree  of  homo- 
geneity. Glass,  as  we  have  seen  in  Chapter  L,  is  not  a 
chemically  homogeneous  substance,  but  rather  a  mixture  of 
a  number  of  substances  of  different  density  and  viscosity. 
Wherever  this  mixture  is  not  sufficiently  intimate,  the 
presence  of  diverse  constituents  becomes  apparent  in  the 
form  of  striae,  arising  from  the  refraction  or  bending  of 
light-rays  as  they  pass  from  one  medium  into  another  of 
different  density.  Except  in  glass  that  has  undergone 
elaborate  stirring  processes,  such  striae  are  never  absent, 
but  the  skill  of  the  glass-maker  consists  in  making 
them  as  few  and  as  minute  as  possible,  and  causing  them 
to  assume  directions  and  positions  in  which  they  shall 
be  as  inconspicuous  as  possible.  In  plate-glass  this  is 
generally  secured  in  a  very  perfect  manner,  and  to  ordinary 
observation  no  striae  are  visible  when  a  piece  of  plate-glass 
is  looked  at  in  the  ordinary  way,  i.e.,  through  its  smallest 
thickness ;  if  the  same  piece  of  glass  be  looked  at  trans- 
versely, the  edges  having  first  been  polished  in  such  a  way 
as  to  render  this  possible,  the  glass  will  be  seen  to  be  full  of 
striae,  generally  running  in  fine  lines  parallel  with  the 


144  GLASS  MANUFACTURE. 

polished  surfaces  of  the  glass.  This  uniform  direction  of 
the  striae  is  partly  derived  from  the  fact  that  the  glass  has 
been  caused  to  flow  in  this  direction  by  the  action  of  the 
roller  when  first  formed  into  a  slab,  but  this  process  would 
not  obliterate  any  serious  inequalities  of  density  which 
might  exist  in  the  glass  as  it  leaves  the  pot,  so  that  success- 
ful results  are  only  attainable  if  great  care  is  taken  to 
secure  the  greatest  possible  homogeneity  in  the  glass 
during  the  melting  process.  - 

At  the  present  time  probably  the  greater  bulk  of  plate- 
glass  is  used  for  the  purpose  of  glazing  windows  of  various 
kinds,  principally  the  show  windows  of  shops,  etc.  As  used 
for  this  purpose  the  glass  is  finished  when  polished  and  cut 
to  size.  The  only  further  manipulation  tbat  is  sometimes 
required  is  that  of  bending  the  glass  to  some  desired 
curvature,  examples  of  bent  plate-glass  window-panes  being 
very  frequently  seen.  This  bending  is  carried  out  on  the 
finished  glass,  i.e.,  after  it  has  been  polished  ;  the  glass  is 
carefully  heated  in  a  special  furnace  until  softened,  and  is 
then  gently  made  to  lie  against  a  stone  or  metal  mould 
which  has  been  provided  with  the  desired  curvature.  It  is 
obvious  that  during  this  operation  there  are  great  risks  of 
spoiling  the  glass ;  roughening  of  the  surface  by  contact 
with  irregular  surfaces  on  either  the  mould,  the  floor  of 
the  kiln,  or  the  implements  used  in  handling  the  glass,  can 
only  be  avoided  by  the  exercise  of  much  skill  and  care, 
while  all  dust  must  also  be  excluded  since  any  particles 
settling  on  the  surface  of  the  hot  glass  would  be  "  burnt  in," 
and  could  not  afterwards  be  detached.  Small  defects  can, 
of  course,  be  subsequently  removed  by  local  hand-polishing, 
and  this  operation  is  nearly  always  resorted  to  where 


ROLLED  OR  PLATE-GLASS.  145 

polished  glass  has  to  undergo  fire-treatment  for  the  purpose 
of  bending. 

In  addition  to  its  use  for  glazing  in  the  ordinary  sense, 
plate-glass  is  employed  for  a  number  of  purposes ;  the  most 
important  and  frequent  of  these  is  in  the  construction  of 
the  better  varieties  of  mirrors.  For  this  purpose  the  glass 
is  frequently  bevelled  at  the  edges,  and  sometimes  a  certain 
amount  of  cutting  is  also  introduced  on  the  face  of  the 
mirror.  Bevelling  is  carried  out  on  special  grinding  and 
polishing  machines,  and  a  great  variety  of  these  are  in  use 
at  the  present  time.  The  process  consists  in  grinding  off 
the  corners  of  the  sheet  of  glass  and  replacing  the  rough 
perpendicular  edge  left  by  the  cutting  diamond  by  a  smooth 
polished  slope  running  down  from  the  front  surface  to  the 
lower  edge  at  an  angle  of  from  45  to  60  degrees.  Since 
only  relatively  small  quantities  of  glass  have  to  be  removed, 
small  grinding  rubbers  only  are  used,  and  in  some  of  the 
latest  machines  these  take  the  form  of  rapidly-revolving 
emery  or  carborundum  wheels.  These  grinding  wheels 
have  proved  so  successful  in  grinding  even  the  hardest 
metals  that  it  is  surprising  to  find  their  use  in  the  glass 
industry  almost  entirely  restricted  to  the  "  cutting  "  of  the 
better  kinds  of  flint  and  "crystal  "  glass  for  table  ware  or 
other  ornamental  purposes.  The  reason  probably  lies  in 
the  fact  that  the  use  of  such  grinding  wheels  results  in  the 
generation  of  a  very  considerable  amount  of  local  heat,  this 
effect  being  intensified  on  account  of  the  low  heat-conduct- 
ing power  of  glass.  If  a  piece  of  glass  be  held  even  lightly 
against  a  rapidly-revolving  emery  wheel  it  will  be  seen  that 
the  part  in  contact  with  the  wheel  is  visibly  red-hot.  This 
local  heating  is  liable  to  lead  to  chipping  and  cracking  of 

G.M.  T. 


146  GLASS  MANUFACTURE. 

the  glass,  and  these  troubles  are  those  actually  experienced 
when  emery  or  carborundum  grinding  is  attempted  on 
larger  pieces  of  glass.  In  the  case  of  at  least  one  modern 
bevel -grinding  machine,  however,  it  is  claimed  that  the 
injurious  effects  of  local  heating  are  avoided  by  carrying 
out  the  entire  operation  under  water. 

For  the  purpose  of  use  in  mirrors,  plate-glass  is  frequently 
silvered,  and  this  process  is  carried  on  so  extensively  that  it 
has  come  to  constitute  an  entire  industry  which  has  no 
essential  connection  with  glass  manufacture  itself;  for 
that  reason  we  do  not  propose  to  enter  on  the  subject  here, 
only  adding  that  the  nature  and  quality  of  the  glass  itself 
considerably  affects  the  ease  and  success  of  the  various 
silvering  processes.  Ordinary  plate-glass,  of  course,  takes 
the  various  silvering  coatings  very  easily  and  uniformly, 
but  there  are  numerous  kinds  of  glass  to  which  this  does 
not  apply,  although  there  are  probably  few  varieties  of 
glass  which  are  sufficiently  stable  for  practical  use,  and  to 
which  a  silvering  coating  cannot  be  satisfactorily  applied, 
provided  that  the  most  suitable  process  be  chosen  in  each 
case. 

While  there  is  little  if  any  use  for  coloured  glass  in  the 
form  of  polished  plate,  entirely  opaque  plate- glass,  coloured 
both  black  and  white,  is  used  for  certain  purposes.  Thus, 
glass  fascias  over  shop-fronts,  the  counters  and  shelves  of 
some  shops,  and  even  tombstones  are  sometimes  made  of 
black  or  white  polished  plate.  From  the  point  of  view  of 
glass  manufacture,  however,  these  varieties  only  differ  from 
ordinary  plate-glass  in  respect  of  certain  additions  to  the 
raw  materials,  resulting  in  the  production  of  the  white  or 
black  opacity.  The  subsequent  treatment  of  the  glass  is 


EOLLED   OR  PLATE-GLASS.  147 

identical  with  that  of  ordinary  plate-glass,  except  that  these 
opaque  varieties  are  rarely  required  to  be  polished  on  both 
sides,  so  that  the  operations  are  simplified  to  that  extent. 

Certain  limitations  to  the  use  of  all  kinds  of  plate-glass, 
whether  rough-rolled,  figured  or  polished,  were  formerly  set 
by  the  fact  that  under  the  influence  of  fire,  partitions  of 
glass  were  liable  to  crack,  splinter  and  fall  to  pieces,  thus 
causing  damage  beyond  their  own  destruction  and  leaving 
a  free  passage  for  the  propagation  of  the  fire.  To  overcome 
these  disadvantages,  glass  manufacturers  have  been  led  to 
introduce  a  network  or  meshing  of  wire  into  the  body  of 
such  glass.  Provided  that  the  glass  and  wire  can  be  made 
so  as  to  unite  properly,  then  the  properties  of  such  rein- 
forced or  "  wired  "  glass  should  be  extremely  valuable.  In 
the  event  of  breakage  from  any  cause,  such  as  fire  or  a 
violent  blow,  while  the  glass  would  still  crack,  the  fragments 
would  be  held  together  by  the  wire  network,  and  the  plates 
of  glass  as  a  whole  would  remain  in  place,  neither  causing 
destruction  through  flying  fragments  nor  allowing  fire  or, 
for  the  matter  of  that,  burglar  a  free  passage.  The  utility 
of  such  a  material  has  been  readily  recognised,  but  the 
difficulty  lies  in  its  production.  These  difficulties  arise 
from  two  causes.  The  most  serious  of  these  is  the  consider- 
able difference  between  the  thermal  expansion  of  the  glass 
and  of  the  wire  to  be  embedded  in  it.  The  wire  is  neces- 
sarily introduced  into  red-hot  glass  while  the  latter  is  being 
rolled  or  cast,  and  therefore  glass  and  wire  have  to  cool 
down  from  a  red  heat  together.  During  this  cooling 
process  the  wire  contracts  much  more  than  the  glass,  and 
breakage  either  results  immediately,  or  the  glass  is  left  in  a 
condition  of  severe  strain  and  is  liable  to  crack  spontaneously 

L  2 


148  GLASS  MANUFACTUEE. 

afterwards.  An  attempt  has  been  made  to  overcome  this 
difficulty  by  using  wire  made  of  a  nickel  steel  alloy,  whose 
thermal  expansion  is  very  similar  to  that  of  glass ;  but,  as  a 
matter  of  fact,  this  similarity  of  thermal  expansion  is  only 
known  to  hold  for  a  short  range  of  moderate  temperatures, 
and  probably  does  not  hold  when  the  steel  alloy  is  heated 
to  redness.  In  another  direction,  greater  success  is  to  be 
attained  by  the  use  of  wire  of  a  very  ductile  metal  which 
should  yield  to  the  stress  that  comes  upon  it  during  cooling  ; 
probably  copper  wire  would  answer  the  purpose,  but  the 
great  cost  of  copper  is  a  deterrent  from  its  use.  A  second 
difficulty  is  met  with  in  introducing  wire  netting  into  glass 
during  the  rolling  operation,  and  this  lies  in  effecting  a 
clean  join  between  glass  and  wire.  Most  metals  when 
heated  give  off  a  considerable  quantity  of  gas,  and  when 
this  gas  is  evolved  after  the  wire  has  been  embedded  in 
glass,  numerous  bubbles  are  formed,  and  these  not  only 
render  the  glass  very  unsightly  but  also  lessen  the  adhesion 
between  the  wire  and  the  glass.  This  difficulty,  however, 
can  be  overcome  more  readily  than  the  first,  since  the 
surface  of  the  metal  can  be  kept  clean  and  the  gas  expelled 
from  the  interior  of  the  wire  by  preliminary  heating.  On 
the  whole,  however,  wired  glass  is  perhaps  still  to  be 
regarded  as  a  product  whose  evolution  is  not  yet  complete, 
and  there  can  be  no  doubt  that  there  are  great  possibilities 
open  to  the  material  when  its  manufacture  has  been  more 
fully  developed. 


CHAPTEK  X. 

SHEET  AND  CROWN  GLASS. 

IN  the  preceding  chapter  we  have  dealt  with  the  processes 
of  manufacture  employed  in  the  production  of  both  the 
crudest  and  the  most  perfect  forms  of  flat  glass  as  used  for 
such  purposes  as  the  glazing  of  window  openings.  The 
products  now  to  be  dealt  with  are  of  an  intermediate 
character,  sheet-glass  possessing  many  of  the  properties  of 
polished  plate,  but  lacking  some  very  important  ones  ;  thus 
sheet-glass  is  sufficiently  transparent  to  allow  an  observer 
to  see  through  it  with  little  or  no  disturbance  — in  the  best 
varieties  of  sheet-glass  the  optical  distortion  caused  by  its 
irregularities  is  so  small  that  the  glass  appears  nearly  as 
perfect  as  polished  plate — but  in  the  cheap  glass  that  is 
used  for  the  glazing  of  ordinary  windows,  sheets  are  often 
employed  which  produce  the  most  disturbing,  and  some- 
times the  most  ludicrous,  distortions  of  objects  seen  through 
them.  It  is  a  curious  fact  that  even  in  good  houses  the 
use  of  such  inferior  glass  is  tolerated  without  comment, 
the  general  public  being,  apparently,  remarkably  non- 
observant  in  this  respect.  In  another  direction  sheet-glass 
has  the  great  advantage  over  plate-glass  that  it  is  very 
much  lighter,  or  can  at  least  be  produced  of  much  smaller 
weight  and  thickness,  although  this  advantage  entails  the 


150  GLASS  MANUFACTURE. 

consequent  disadvantage  that  sheet-glass  is  usually  much 
weaker  than  plate,  and  can  only  be  used  in  much  smaller 
sizes.  In  recent  times  the  production  of  relatively  thin 
plate-glass  has,  however,  made  such  strides  that  it  is  now 
possible  to  obtain  polished  plate-glass  thin  enough  and 
light  enough  for  almost  every  architectural  purpose. 
Finally,  the  most  important  advantage  of  sheet-glass,  and 
the  one  which  alone  secures  its  use  in  a  great  number  of 
cases  in  preference  to  plate-glass,  is  its  cheapness,  the  price 
of  ordinary  sheet-glass  being  about  one-fourth  that  of 
plate-glass  of  the  same  size. 

The  raw  materials  for  the  manufacture  of  sheet-glass  are 
sand,  limestone,  salt-cake,  and  a  few  accessory  substances, 
such  as  arsenic,  oxide  of  manganese,  anthracite  coal  or 
coke,  which  differ  considerably  according  to  the  practice  of 
each  particular  works.  In  a  general  way  these  materials 
have  already  been  dealt  with  in  Chapter  III.,  and  we  need 
only  add  here  that  the  sheet-glass  manufacturer  must  keep 
in  view  two  decidedly  conflicting  considerations.  On  the 
one  hand  the  requirements  made  in  the  case  of  sheet-glass 
as  regards  colour  and  purity  render  a  rigorous  choice  of 
raw  material  and  the  exclusion  of  anything  at  all  doubtful 
very  desirable ;  but  on  the  other  hand  the  chief  com- 
mercial consideration  in  connection  with  this  product  is  its 
cheapness,  and  in  order  to  maintain  a  low  selling  price  at  a 
profit  to  himself  the  manufacturer  must  rigorously  exclude 
all  expensive  raw  materials.  For  this  reason  sheet-glass 
works  such  as  those  of  Belgium  and  some  parts  of  Germany, 
which  have  large  deposits  of  pure  sand  close  at  hand, 
possess  a  very  considerable  advantage  over  those  in  less 
favoured  situations,  since  sand  in  particular  forms  so  large 


SHEET  AND   CEOWN   GLASS.  151 

a  proportion  of  the  glass,  and  the  cost  of  carriage  frequently 
exceeds,  and  in  many  cases  very  greatly  exceeds,  the  actual 
price  of  the  sand  itself.  The  same  considerations  will  apply, 
although  in  somewhat  lesser  degree,  to  the  other  bulky 
materials,  such  as  limestone  and  salt-cake ;  but  both  these 
are  more  generally  obtainable  at  moderate  prices  than 
are  glass-making  sands  of  adequate  quality  for  sheet 
manufacture. 

Ordinary  "  white  "  sheet-glass  is  now  almost  universally 
produced  in  tank  furnaces,  and  a  very  great  variety  of 
these  furnaces  are  used  or  advocated  for  the  purpose.  It 
would  be  beyond  the  scope  of  the  present  book  to  enter  in 
detail  into  the  construction  of  these  various  types  of  furnace 
or  to  discuss  their  relative  merits  at  length.  Only  a  brief 
outline  of  the  chief  characteristics  of  the  most  important 
forms  of  sheet-tank  furnaces  will  therefore  be  given 
here. 

Sheet  tanks  differ  from  each  other  in  several  important 
respects ;  these  relate  to  the  sub-division  of  the  tank  into 
one,  two,  or  even  three  more  or  less  separate  chambers,  to 
the  depth  of  the  bath  of  molten  glass  and  the  height  of  the 
"crown"  or  vault  of  the  furnace  chamber,  to  the  shape 
and  position  of  the  apertures  by  which  the  gas  and  air  are 
admitted  into  the  furnace,  and  the  resultant  shape  and 
disposition  of  the  flame,  and  finally  to  the  position  and 
arrangement  of  the  regenerative  appliances  by  which  some 
of  the  heat  of  the  waste  gases  is  returned  into  the 
furnace. 

Taking  these  principal  points  in  order,  we  find  that  in  some 
sheet  tank  furnaces  the  whole  furnace  constitutes  a  single 
large  chamber.  In  this  type  of  furnace  the  whole  process 


152  GLASS  MANUFACTUEE. 

of  fusion  and  fining  of  the  glass  goes  on  in  this  single 
chamber,  and  an  endeavour  is  made  to  graduate  the 
temperature  of  the  furnace  in  a  suitable  manner  from  the 
hot  end  where  the  raw  materials  have  to  be  melted  down 
to  the  colder  end  where  the  glass  must  be  sufficiently 
viscous  to  be  gathered  on  the  pipes.  It  is  obvious  that 
this  control  of  the  temperature  cannot  be  so  perfect  in  a 
furnace  of  the  single  chamber  type  as  in  one  that  is 
sub-divided.  .  Such  sub-divided  furnaces  are,  as  a  matter  of 
fact,  much  more  frequent  in  sheet-glass  practice ;  but  this 
practice  differs  widely  as  to  the  manner  and  degree  of 
the  sub-division  introduced.  In  the  extreme  form  the 
glass  practically  passes  through  three  independent  furnaces 
merely  connected  with  one  another  by  suitable  openings  of 
relatively  small  area  through  which  the  glass  flows  from 
one  to  the  other.  If  it  were  possible  to  build  furnaces  of 
materials  that  could  resist  the  action  of  heat  and  of  molten 
glass  to  an  indefinite  extent,  it  is  probable  that  this  extreme 
type  would  prove  the  best,  since  it  gives  the  operator  of  the 
furnace  the  means  of  controlling  the  flow  of  glass  in  such  a 
way  that  no  unmelted  material  can  leave  the  melting 
chamber  and  enter  the  fining  chamber,  and  that  no 
insufficiently  fined  glass  can  leave  the  fining  chamber  and 
find  its  way  into  the  working  chamber.  But  in  practice 
the  fact  that  this  extreme  sub-division  introduces  a  great 
deal  of  extra  furnace  wall,  exposed  both  to  heat  and  to 
contact  with  the  glass,  involves  very  serious  compensating 
disadvantages — the  cost  of  construction,  maintenance  and 
renewal  of  the  furnace  is  greatly  increased,  while  there 
is  also  an  increased  source  of  contamination  of  the  glass 
from  the  erosion  of  the  furnace  walls.  It  is,  therefore,  in 


SHEET  AND   CROWN  GLASS.  153 

accordance  with  expectations  to  find  that  the  most  successful 
furnaces  for  the  production  of  sheet-glass  are  intermediate 
in  this  respect  between  the  simple  open  furnace  and  the 
completely  sub-divided  one.  In  some  cases  the  working 
chamber  is  separated  from  the  melting  and  fining  chamber 
by  a  transverse  wall  above  the  level  of  the  glass,  while  fire- 
clay blocks  floating  in  the  glass  just  below  this  cross  wall 
serve  to  complete  the  separation  and  to  retain  any  surface 
impurities  that  may  float  down  the  furnace. 

As  regards  the  depth  of  glass  in  the  tank,  practice 
also  varies  very  much.  The  advantages  claimed  for  a  deep 
bath  are  that  the  fire-clay  bottom  of  the  furnace  is  thereby 
kept  colder  and  is  consequently  less  attacked,  so  that  this 
portion  of  the  furnace  will  last  for  many  years.  On  the 
other  hand  the  existence  of  a  great  mass  of  glass  at  a 
moderate  heat  may  easily  prove  the  source  of  contamina- 
tion arising  from  crystallisation  or  "  devitrification " 
occurring  there  and  spreading  into  the  hotter  glass  above. 
Also,  if  for  any  reason  it  should  become  necessary  to 
remove  part  or  all  of  the  contents  of  the  tank,  the  greater 
mass  of  glass  in  those  with  deep  baths  becomes  a  formi- 
dable obstacle.  On  the  whole,  however,  modern  practice 
appears  to  favour  the  use  of  deeper  baths,  depths  of 
2  ft.  6  in.  or  even  3  ft.  being  very  usual,  while  depths  up  to 
4  ft.  have  been  used. 

The  question  of  the  proper  height  of  the  "  crown  "  or 
vault  of  the  furnace  is  of  considerable  importance  to  the 
proper  working  of  the  tank.  For  the  purpose  of  producing 
the  most  perfect  combustion,  it  is  now  contended  that  a 
large  free  flame-space  is  required.  The  earlier  glass-melting 
tanks,  like  the  earlier  steel  furnaces,  were  built  with  very 


154  GLASS   MANUFACTURE. 

low  crowns,  forcing  the  flame  into  contact  with  the  surface 
of  the  molten  glass,  the  object  being  to  promote  direct 
heating  by  immediate  contact  of  flame  and  glass ;  the 
modern  tendency,  however,  is  strongly  in  the  direction  of 
higher  crowns,  leaving  the  heating  of  the  glass  to  be 
accomplished  by  radiation  rather  than  direct  conduction  of 
heat.  There  can  be  little  doubt  that  up  to  a  certain  point 
the  enlargement  of  the  flame-space  tends  towards  greater 
cleanliness  of  working  and  a  certain  economy  of  fuel,  but  if 
the  height  of  a  furnace  crown  be  excessive  there  is  a  decided 
loss  of  economy.  Flame- spaces  as  high  as  6  ft.  from  the 
level  of  the  glass  to  the  highest  part  of  the  crown  have  been 
used,  but  the  more  usual  heights  range  from  2  ft.  to  5  ft. 

The  "  ports  "  or  apertures  by  which  pre-heated  gas  and 
air  enter  the  furnace  chamber  differ  very  widely  in  various 
furnaces.  In  some  cases  the  gas  and  air  are  allowed  to 
meet  in  a  small  combustion  chamber  just  before  entering 
the  furnace  itself,  while  in  other  cases  the  gas  and  air  enter 
the  furnace  by  entirely  separate  openings,  only  meeting  in 
the  furnace  chamber.  The  latter  arrangement  tends  to  the 
formation  of  a  highly  reducing  flame,  which  is  advan- 
tageous for  the  reduction  of  salt-cake,  but  is  by  no  means 
economical  as  regards  fuel  consumption.  On  the  other 
hand,  by  producing  a  perfect  mixing  of  the  entering  gas 
and  air  in  suitable  proportions,  the  other  type  of  ports  can 
be  made  to  give  almost  any  kind  of  flame  desired,  although 
their  tendency  is  to  form  a  more  oxidising  atmosphere 
within  the  furnace.  The  latter  type  of  ports,  although 
widely  varied  in  detail,  are  now  almost  universally  adopted 
in  sheet  tank  furnaces. 

All  modern  tank  furnaces  work  on  the  principle  of  the 


SHEET   AND   CEOWN   GLASS.  155 

recovery  of  heat  from  the  heated  products  of  combustion  as 
they  leave  the  furnace,  and  the  return  of  this  heat  to  the 
furnace  by  utilising  it  to  pre-heat  the  incoming  gas  and  air  ; 
but  the  means  employed  to  effect  the  application  of  this 
"regenerative"  principle  differ  considerably  in  various 
types  of  plant.  Perhaps  the  most  widely-used  form  of 
furnace  is  the  direct  descendant  of  the  original  Siemens 
regenerative  furnace,  in  which  four  regenerator  chambers 
are  provided  with  means  for  reversing  the  flow  of  gas  and 
air  in  such  a  way  that  each  pair  of  chambers  serves 
alternately  to  absorb  the  heat  of  the  outgoing  gases  and 
subsequently  to  return  this  heat  to  the  incoming  air  that 
passes  through  one,  and  the  incoming  gas  that  passes 
through  the  other  of  these  chambers.  In  these  furnaces, 
the  regenerator  chambers  themselves  are  generally  placed 
underneath  the  melting  furnace,  and  they  are  built  of 
fire-brick  and  filled  with  loosely-stacked  fire-bricks,  whose 
function  it  is  to  absorb  or  deliver  the  heat.  In  the  most 
modern  type  of  furnaces  of  this  class,  the  gas- regenerators 
are  omitted  entirely,  the  air  only  being  pre-heated  by  means 
of  regenerators,  while  the  gas  enters  the  furnace  direct  from 
the  producer,  thus  carrying  with  it  the  heat  generated  in 
the  producer  during  the  gasification  of  the  fuel.  While 
this  arrangement  is  undoubtedly  economical,  it  has  the 
serious  disadvantage,  especially  in  the  manufacture  of  sheet- 
glass,  that  the  gas,  rushing  direct  from  the  producer  into 
the  furnace,  carries  with  it  a  great  deal  of  dust  and  ash, 
which  it  has  no  opportunity  of  depositing,  as  in  the  older 
types  of  furnace,  in  long  flues. 

The  most  serious  disadvantages  of  the  ordinary  types  of 
regenerative  furnaces  are  due  to  the  considerable  dimensions 


156  GLASS  MANUFACTUEE. 

of  the  regenerative  apparatus,  necessitating  a  costly  form  of 
construction  and  occupying  a  large  space,  while  the 
necessity  of  periodically  reversing  the  valves  so  as  to  secure 
the  alternation  in  the  flow  of  outgoing  and  incoming  gases 
requires  special  attention  on  the  part  of  the  men  engaged 
in  operating  the  furnace,  as  well  as  the  construction  and 
maintenance  of  valves  under  conditions  of  heat  and  dirt 
that  are  not  favourable  to  the  life  of  mechanical  appliances. 
It  is  claimed  that  all  these  disadvantages  are  overcome  to  a 
considerable  extent  in  one  or  other  of  the  various  forms  of 
furnace  known  as  "  recuperative."  In  these  furnaces  there 
is  no  alternation  of  flow,  and  the  regenerator  chambers  are 
replaced  by  the  "  recuperators."  These  consist  of  a  large 
number  of  small  flues  or  pipes  passing  through  a  built-up 
mass  of  fire-brick  in  two  directions  at  right-angles  to  one 
another ;  through  the  pipes  running  in  one  direction  the 
waste  gases  pass  out  to  the  chimney,  while  the  incoming 
gas  and  air  pass  through  the  other  set  of  pipes.  A  trans- 
ference of  heat  between  the  two  currents  of  gas  takes  place 
by  the  conductivity  of  the  fire-brick,  and  thus  the  outgoing 
gases  are  continuously  cooled  while  the  ingoing  gases  are 
heated — the  transference  of  heat  being  somewhat  similar  to 
that  which  takes  place  in  the  surface  condenser  of  a  steam 
engine.  Theoretically  this  is  a  much  simpler  arrangement 
than  that  of  separate  regenerator  chambers,  and  to  some 
extent  it  is  found  preferable  in  practice,  but  there  are 
certain  disadvantages  associated  with  the  system  which 
arise  principally  from  the  peculiar  nature  of  the  material — 
fire-brick — of  which  the  recuperators  must  be  constructed. 
In  the  first  place,  the  heat-conductivity  of  fire-brick  is  not 
very  high,  so  that,  in  order  to  secure  efficiency,  the  recupe- 


SHEET  AND   CEOWN  GLASS.  157 

rators  must  be  large,  and  while  the  individual  pipes  must  be 
of  small  diameter,  their  area  as  a  whole  must  be  large 
enough  to  allow  the  gases  to  pass  through  somewhat  slowly. 
Next,  owing  to  the  tendency  of  fire-brick  to  warp, 
shrink  and  crack  under  the  prolonged  effects  of  high 
temperatures,  it  becomes  difficult  to  prevent  leakage  of 
gases  from  one  set  of  pipes  into  the  other.  If  this  occurs 
to  a  moderate  extent  its  only  effect  will  be  to  allow  some  of 
the  combustible  gas  to  pass  direct  to  the  chimney,  and  at 
the  same  time  a  dilution  of  the  gases  entering  the  furnace 
by  an  addition  of  products  of  combustion  from  the  waste- 
gas  flues.  This,  of  course,  will  materially  reduce  the 
efficiency  of  the  furnace  and  require  a  higher  fuel  con- 
sumption if  the  temperature  of  the  furnace  is  to  be 
maintained  at  its  proper  level.  If,  however,  the  leakage 
should  become  more  serious,  a  disastrous  explosion  might 
easily  result,  particularly  if  the  nature  of  the  leakage  were 
such  as  to  allow  the  incoming  gas  and  air  to  mix  in  the 
flues.  It  follows  from  these  considerations  that,  although 
the  recuperative  furnace  is  somewhat  simpler  and  cheaper 
to  construct,  it  requires,  if  anything,  more  careful  main- 
tenance than  the  older  forms  of  regenerative  furnace. 

Tank  furnaces  for  the  production  of  sheet-glass  in  this 
country  are  generally  worked  from  early  on  Monday  morn- 
ing until  late  on  Saturday  night,  glass-blowing  operations 
being  suspended  during  Sunday,  although  the  heat  of  the 
furnace  must  be  maintained.  On  the  Continent,  and 
especially  in  Belgium,  the  work  in  connection  with  these 
furnaces  goes  on  without  any  intermission  on  Sunday — a 
difference  which,  however  desirable  the  English  practice 
may  be,  has  the  effect  of  handicapping  the  outpu  of  a 


158  GLASS  MANUFACTUKE. 

British  furnace  of  equal  capacity  by  about    10  per  cent, 
without  materially  lessening  the  working  cost. 

The  process  of  blowing  sheet-glass  in  an  English  glass- 
works is  generally  carried  out  by  groups  of  three  workmen, 
viz.,  a  "pipe-warmer,"  a  "gatherer"  and  a  "blower," 
although  the  precise  division  of  the  work  varies  according 
to  circumstances.  The  pipe-warmer's  work  consists  in  the 
first  place  in  fetching  the  blowing-pipe  from  a  small  sub- 
sidiary furnace  in  which  he  has  previously  ^placed  it  for  the 
purpose  of  warmin  up  the  thick  "nose"  end  upon  which 
the  glass  is  subsequently  gathered.  The  sheet-blower's  pipe 
itself  is  an  iron  tube  about  4  ft.  6  in.  long,  provided  at 
the  one  end  with  a  wooden  sleeve  or  handle,  and  a  mouth- 
piece, while  the  other  end  is  thickened  up  into  a  substantial 
cone,  having  a  round  end.  Before  introducing  the  pipe 
into  the  opening  of  the  tank  furnace,  the  pipe-warmer  must 
see  that  the  hot  end  of  the  pipe  is  free  from  scale  or  dirt 
and  must  test,  by  blowing  through  it,  whether  the  pipe  is 
free  from  internal  obstructions.  He  then  places  the  butt 
of  the  pipe  in  the  opening  of  the  furnace  and  allows  it  to 
acquire  as  nearly  as  possible  the  temperature  of  the  molten 
glass.  When  this  is  the  case  the  pipe  is  either  handed  on 
to  the  gatherer,  or  the  pipe-warmer,  who  is  usually  only  a 
youth,  may  take  the  process  one  step  further  before  handing 
it  on  to  the  more  highly  skilled  workman.  This  next  step 
consists  in  taking  up  the  first  gathering  of  glass  on  the 
pipe.  For  this  purpose  the  hot  nose  of  the  pipe  is  dipped 
into  the  molten  glass,  turned  slowly  round  once  or  twice 
and  then  removed,  the  thread  of  viscous  glass  that  comes 
up  with  the  pipe  being  cut  off  against  the  fire-clay  ring 
that  floats  in  the  glass  in  front  of  the  working  opening. 


SHEET  AND   CEOWN   GLASS.  159 

A  small  quantity  of  glass  is  thus  left  adhering  to  the 
nose  of  the  pipe,  and  this  is  now  allowed  to  cool  down 
until  it  is  fairly  stiff,  the  whole  pipe  being  meanwhile 
rotated  so  as  to  keep  this  first  gathering  nicely  rounded, 
while  a  slight  application  of  air-pressure,  by  blowing 
down  the  pipe,  forms  a  very  small  hollow  space  in  the 
mass  of  glass  and  secures  the  freedom  of  the  opening  of 
the  pipe.  When  the  glass  forming  the  first  gathering  has 
cooled  sufficiently,  the  gatherer  proceeds  to  take  up  the 
second  gathering  upon  it.  The  pipe  is  again  introduced 
into  the  furnace  and  gradually  dipped  into  the  molten 
glass,  but  this  must  be  done  with  great  care  so  as  to  avoid 
the  inclusion  of  air-bells  between  the  glass  already  on  the 
pipe  and  the  new  layer  of  hotter  glass  that  is  now  taken  up. 
This  freedom  from  air-bells  is  secured  by  a  skilful  gatherer 
by  a  gradual  rotation  of  the  pipe  as  it  is  lowered  into  the 
glass,  thus  allowing  the  two  layers  of  glass  to  come  into 
contact  with  a  sort  of  rolling  motion  that  allows  the  air 
time  to  escape.  When  completely  immersed,  the  pipe  is 
rotated  a  few  times  and  is  then  withdrawn  and  the  "  thread" 
again  cut  off.  The  mass  of  glass  on  the  end  of  the  pipe  is 
now  considerably  larger  than  before  and  requires  more 
careful  manipulation  to  cause  it  to  retain  the  proper,  nearly 
spherical,  shape.  During  the  cooling  process  which  now 
follows  the  pipe  is  laid  across  an  iron  trough,  kept  brim- 
ful of  water;  this  serves  to  cool  the  pipe  itself,  and  also 
allows  the  pipe  to  be  readily  rotated  backwards  and 
forwards  by  rolling  it  a  little  way  along  the  trough.  When 
the  whole  mass  of  glass  has  again  cooled  sufficiently  to  be 
manipulated  without  risk  of  rapid  deformation,  a  third 
gathering  of  glass  is  taken  up,  in  precisely  the  same  manner 


160  GLASS   MANUFACTUKE. 

as  that  already  described  for  the  second  gathering,  and  if 
the  quantity  of  glass  required  is  large,  or  the  glass  itself  is 
so  hot  and  fluid  that  only  a  comparatively  small  weight 
adheres  at  each  time  of  gathering,  the  process  may  be 
repeated  a  fourth  or  even  a  fifth  time,  but  as  the  weight  of 
pipe  and  adhering  glass  increases  with  each  gathering,  each 
step  becomes  more  laborious,  while  the  hot  glass,  being 
now  held  on  a  much  larger  sphere,  tends  to  flow  off  more 
readily,  so  that  greater  skill  is  required  to  avoid  "  losing" 
the  gathering. 

The  care  and  skill  with  which  these  operations  of 
gathering  are  carried  out  determine,  to  a  large  extent,  the 
quality  of  the  resulting  sheet  of  glass ;  any  want  of 
regularity  in  the  shape  of  the  gathering  leads  inevitably  to 
variations  of  thickness  in  different  parts  of  the  sheet,  while 
careless  gathering  will  introduce  bubbles  or  "  blisters  "  and 
other  markings.  During  the  intermediate  cooling  stages 
the  glass  must  be  protected  from  dust  and  dirt  of  all  kinds, 
since  minute  specks  falling  upon  the  hot  glass  give  rise 
an  evolution  of  minute  gas  bubbles  which  become  painfulb 
evident  in  the  sorting  room. 

When  the  last  gathering  has  been  taken  up  and  the  mass 
cooled  so  far  as  to  allow  of  its  being  carried  about  without 
fear  of  loss,  the  glass  forms  an  approximately  spherical 
mass,  with  the  nose-end  of  the  pipe  at  or  near  the  centre  of 
the  sphere.  The  next  stages  of  the  process  consist  in  the 
preliminary  shaping  of  this  mass  in  such  a  way  as  to  bring 
the  bulk  of  the  glass  beyond  the  end  of  the  pipe,  and  then 
in  forming  just  beyond  the  end  of  the  pipe  a  widened  shoulder 
of  thinner  and  therefore  colder  glass,  of  the  diameter 
required  for  the  cylinder  into  which  the  glass  is  to  be 


SHEET  AND  CEOWN  GLASS.  161 

blown.  This  is  done  by  bringing  the  glass  into  the  succes- 
sive shapes  shown  in  Fig.  12,  the  forming  of  the  glass 
being  effected  by  the  aid  of  specially  shaped  blocks  and 
other  shaping  instruments  in  which  the  glass  is  turned  and 
blown.  The  final  shape  attained  at  this  stage  is  a  squat 
cylinder  containing  the  bulk  of  the  glass  at  its  lower  end, 
and  connected  to  the  pipe  by  the  thinner  and  colder  neck 
and  shoulder  already  mentioned. 

At  this  point  of  the  process  the  pipe  with  its  adherent 
glass  is  handed  over  to  the  blower  proper.     This  operator 


FIG.  12.— Early  stages  in  the  formation  of  cylinders  for  sheet  glass. 

works  on  a  special  stage  erected  in  front  of  small  furnaces, 
called  "  blowing  holes,"  although  in  some  works  these  are 
dispensed  with,  and  the  stages  are  erected  in  front  of  the 
melting  furnace  itself.  The  sheet-blower's  stage  is  simply 
a  platform  placed  over  or  at  the  side  of  a  suitable  excava- 
tion which  gives  the  blower  the  necessary  space  to  swing 
the  pipe  and  cylinder  freely  at  arm's  length.  The  blowing 
process  itself  involves  very  little  actual  blowing,  but  depends 
rather  upon  the  action  of  gravitation  and  on  centrifugal 
effects  for  the  formation  of  the  large,  elongated  cylinder 
from  the  squat  cylinder  with  which  the  blower  com- 
mences. The  process  consists  in  holding  the  thick,  lower 
G.M.  M 


162  GLASS  MANUFACTURE. 

end  of  the  cylinder  in  the  heating-furnace,  and  when  suffi- 
ciently hot,  withdrawing  it  and  swinging  the  pipe  with  a 
pendulum  movement  in  the  blower's  pit.  The  cylinder 
thus  elongates  itself  under  its  own  weight,  and  any  ten- 
dency to  collapse  is  counteracted  by  the  application  of  air- 
pressure  by  the  mouth,  the  pipe  being  also,  at  times, 
rotated  rapidly  about  its  own  axis.  The  re-heating  of  the 


FIG.  13. — Later  stage  in  sheet  glass  blowing. 

lower  end  of  the  cylinder  is  repeated  several  times,  until 
finally  the  glass  has  assumed  the  form  of  a  cylinder  of  equal 
thickness  all  over,  but  closed  with  a  rounded  dome  at  the 
lower  end  (Fig.  13).  This  rounded  end  is  now  opened. 
In  the  case  of  fairly  thin  and  light  cylinders  this  is  done  by 
holding  the  thumb  over  the  mouthpiece  of  the  pipe  in  such 
a  way  as  to  make  an  air-tight  seal,  and  then  heating  the 
end  of  the  cylinder  in  the  blowing-hole.  The  heat  both 
softens  the  glass  at  the  end  and  at  the  same  time  causes 


SHEET  AND   CROWN   GLASS.  163 

considerable  expansion  of  the  air  enclosed  in  the  cylinder, 
with  the  result  that  the  end  of  the  cylinder  is  burst  open. 
After  a  little  further  heating,  during  which  the  glass  at  the 
end  of  the  cylinder  becomes  very  soft,  and  takes  a  wavy, 
curly  shape,  the  blower  withdraws  the  cylinder  from  the 
furnace,  and  holding  it  vertically  downwards  in  his  pit, 
spins  it  rapidly  about  its  longitudinal  axis.  The  soft  glass 
at  the  lower  end  immediately  opens  out  under  the  centri- 
fugal action,  and  the  blower  increases  the  speed  of  rotation 
until  the  soft  glass  has  opened  out  far  enough  to  form  a 
true  continuation  of  the  rest  of  the  cylinder,  and  in  this 
position  it  is  allowed  to  solidify.  With  thick,  heavy 
cylinders,  the  first  opening  of  the  end  is  done  in  a  different 
way.  A  small  quantity  of  hot  glass  is  taken  up  by  an 
assistant  on  an  iron  rod,  and  is  laid  upon  the  centre  of  the 
closed  end  of  the  cylinder.  The  heat  of  this  mass  of  hot 
glass  softens  the  glass  of  the  cylinder,  and  the  operator, 
with  the  aid  of  a  special  pair  of  shears,  cuts  out  a  small 
circle  of  this  softened  glass,  thus  opening  the  end  of  the 
cylinder.  The  final  operation  of  straightening  out  the 
opened  end  is  carried  out  in  the  same  way  as  described 
above  for  lighter  cylinders. 

The  completed  cylinder,  still  attached  to  the  pipe,  is  now 
carried  away  from  the  blowing-stage  and  laid  upon  a 
wooden  rack  ;  then  the  blower  takes  up  a  piece  of  cold  iron, 
and  placing  it  against  the  neck  of  glass  attaching  the 
cylinder  to  the  pipe,  produces  a  crack  ;  a  short  jerk  then 
serves  to  completely  sever  the  pipe  from  the  cylinder.  A 
boy  now  takes  the  pipe  to  a  stand  where  it  is  allowed  to 
cool  and  where  the  adhering  glass  cracks  off  from  it  prior 
to  passing  it  back  to  the  pipe-warmer  for  fresh  use. 

M2 


164  GLASS  MANUFACTUEE. 

On  the  wooden  rack  the  cylinder  of  glass  is  allowed  to 
cool  to  a  certain  extent,  and  then  the  remaining  portion  of 
the  neck  and  shoulder  (see  Fig.  13)  are  removed.  This  is 
done  by  a  boy  who  passes  a  thread  of  soft,  hot  glass 
around  the  cylinder  at  the  point  where  it  is  to  be  cut  off ; 
the  thread  of  hot  glass  merely  serves  to  produce  intense 
local  heating,  for  as  soon  as  it  has  become  stiff,  the  thread 
of  glass  is  pushed  off  and  a  cold  or  moist  iron  is  applied  to 
the  cylinder  at  the  point  where  it  had  been  heated  by  the 
thread.  As  a  rule  a  crack  immediately  runs  completely 
round  the  cylinder  along  the  line  of  the  thread,  and  the 
"cap"  is  thus  removed.  The  glass  is  now  in  the  form  of 
a  uniform  cylinder  open  at  both  ends,  but  it  must  be 
opened  out  into  a  flat  sheet  before  it  can  assume  the  familiar 
form  of  sheet-glass. 

The  first  stage  in  the  opening-out  process  is  that  of 
splitting.  For  this  purpose  the  cylinders  are  carried  to  a 
special  stand,  upon  which  they  are  laid  in  a  horizontal 
position,  and  here  a  crack  or  cut  is  made  along  one  of  the 
generating-lines  of  the  cylinder.  This  may  be  done  either 
by  the  application  of  a  hot  iron,  followed,  if  necessary,  by 
slight  moistening,  or  by  the  aid  of  a  cut  from  a  heavy 
diamond  drawn  skilfully  down  the  inside  of  the  cylinder. 
It  will  be  seen  from  the  account  of  the  process  so  far  given, 
that  the  glass  has  as  yet  undergone  no  real  annealing, 
although  the  blower  is  expected  to  "  anneal "  his  cylinder 
during  the  blowing  process,  as  far  as  possible,  by  never 
allowing  it  to  cool  too  suddenly,  and  this  degree  of  annealing 
is  usually  sufficient  to  save  the  cylinder  from  breaking 
under  its  internal  stresses  when  left  to  cool  on  the  racks. 
The  surface  of  the  glass,  however,  is  left  in  a  decidedly 


SHEET  AND   CKOWN  GLASS.  165 

hardened  condition,  especially  on  the  outside,  which  has 
necessarily  been  most  rapidly  cooled.  For  this  reason — 
among  others — the  splitting  cut  is  always  made  on  the 
inside  of  the  cylinder.  The  difference  between  the  rates  of 
cooling  of  the  outside  and  inside  of  the  cylinder  has  a 
further  effect,  which  becomes  evident  as  soon  as  the  cylinder 
is  split.  The  outside  having  become  hard  while  the  inside 
was  still  relatively  soft,  the  outer  layers  of  glass  are  in  a 
state  of  compression  and  the  inner  layers  in  a  state  of 
tension  in  the  cold  cylinder.  As  soon  as  the  cylinder  is 
split,  however,  these  stresses  are  to  some  extent  relieved, 
the  inner  layers  being  then  free  to  contract  and  the  outer 
layers  to  expand  ;  the  result  is  an  increase  in  the  curvature 
of  the  cylinder,  which  slightly  decreases  in  diameter,  the  cut 
edges  overlapping.  If  the  cylinder  has  been  cooled  rather 
too  quickly,  or  if  the  glass  itself  has  a  high  co-efficient  of 
expansion,  this  release  of  internal  stresses  at  the  moment 
of  splitting  becomes  very  marked,  and  each  cylinder  splits 
with  the  sound  of  a  small  explosion,  while  if  the  internal 
stresses  are  still  more  severe,  the  cylinders  may  even  fly  to 
pieces  as  soon  as  they  are  cut. 

The  next  stage  in  the  manufacture  of  a  sheet  of  glass  is 
the  flattening  and  annealing  process.  For  this  purpose  the 
split  cylinders  are  taken  to  a  special  kiln,  generally  known 
as  a  "lear,"  or  "lehr,"  where  they  are  first  of  all  raised  to  a 
dull  red-heat;  they  are  then  lifted,  one  at  a  time,  on  to  a 
smooth  stone  or  slab  placed  in  a  chamber  of  the  kiln  where 
the  heat  is  great  enough  to  soften  the  glass.  Here  the 
cylinder  is  laid  down  with  the  split  edges  upwards,  and  by 
means  of  a  wooden  tool  the  glass  is  slowly  spread  out,  being 
finally  rubbed  down  into  perfect  contact  with  the  slab  or 


166  GLASS  MANUFACTURE. 

"  lagre."  From  the  flattening  slab,  the  sheet  as  it  now  is 
passes  into  the  annealing  kiln,  which  communicates  with 
the  flattening  chamber.  This  consists,  similarly  to  other 
continuous  annealing  kilns  already  described  in  connection 
with  other  varieties  of  glass,  of  a  long  tunnel,  heated  to  the 
temperature  of  the  flattening  kiln  at  one  end  and  nearly 
cold  at  the  other.  The  sheets  are  moved  down  this  tunnel 
at  a  uniform  slow  rate  by  the  action  of  a  system  of  grids 
which,  at  intervals,  lift  the  sheets  from  the  bottom  of  the 
kiln,  move  them  forward  by  a  short  distance,  and  again 
deposit  them  on  the  bottom,  the  grids  themselves  returning 
to  their  former  position  by  a  retrograde  movement  made 
below  the  level  of  the  kiln -bottom,  and  therefore  not 
affecting  the  glass. 

On  leaving  the  annealing  kiln  the  sheets  of  glass  are 
sometimes  covered  with  a  white  deposit  arising  from  the 
products  of  combustion  in  the  kiln  and  their  interaction 
with  the  glass  itself.  This  deposit  can  be  removed  by 
simple  mechanical  rubbing,  but  it  is  usual  to  dip  the  glass 
into  a  weak  acid  bath,  which  dissolves  the  white  film  and 
leaves  the  glass  clear  and  bright,  ready  for  use. 

From  the  annealing  kiln  the  finished  sheets  of  glass  are 
taken  to  the  sorting  room,  where  they  are  examined  in  a 
good  light  against  a  black  background,  and  are  sorted 
according  to  their  quality  for  different  purposes. 

The  defects  which  are  found  in  sheet-glass  are  of  a  very 
varied  nature,  as  would  bo  anticipated  from  the  long  and 
complicated  process  of  manufacture  which  the  material 
undergoes  in  the  course  of  its  transformation  from  the  raw 
materials  into  the  finished  sheet  of  glass.  A  full  enumera- 
tion of  all  possible  defects,  with  their  technical  names,  need 


SHEET  AND   CEOWN   GLASS.  167 

not  be  given  here,  but  a  description  of  the  more  important 
and  frequent  ones  will  be  useful.  The  defects  may  be 
conveniently  grouped  according  to  the  stage  of  the  process 
from  which  they  originate. 

The  first  class  of  defects  accordingly  embraces  those  that 
arise  from  the  condition  of  the  glass  as  it  exists  in  the 
working-end  of  the  furnace.  Chief  of  these  are  white 
opaque  enclosures,  known  as  "  stones."  These  may  arise 
from  a  variety  of  causes  within  the  furnace,  such  as  an 
admixture  of  infusible  impurities  with  the  raw  materials, 
insufficient  heat  or  duration  of  melting,  leading  to  a  residue 
of  unmelted  raw  material  in  the  finished  glass,  or  from 
defective  condition  of  the  interior  of  the  furnace,  leading  to 
contamination  of  the  glass  with  small  particles  of  fire- 
brick. Further,  if  any  part  of  the  furnace  has  been 
allowed  to  remain  at  too  low  a  temperature,  or  if  the 
composition  of  the  glass  is  unsuitable,  crystallisation 
may  occur  in  the  glass,  and  white  patches  of  crystalline 
material  may  find  their  way  into  the  finished  sheets. 
Another  defect  that  may  arise  from  the  condition 
of  the  glass  in  the  furnace  is  the  presence  of  numerous 
small  bubbles,  known  as  "seed"  in  the  glass.  By  the 
blowing  process  these  are  drawn  out  into  pointed  ovals,  and 
they  are  rarely  quite  absent  from  sheet-glass.  They  arise 
from  either  incomplete  fining  of  the  glass  in  the  furnace  or 
from  allowing  the  glass  to  come  into  contact  with  minute 
particles  of  dust  during  the  gathering  process.  Another 
possible  defect  to  the  glass  itself  may  be  found  at  times  in 
too  deep  a  colour.  This  is  only  seen  readily  when  a  sheet 
of  some  size  is  examined  edgewise,  as  most  varieties  of 
ordinary  sheet -glass  are  too  free  from  colour  to  allow  this 


16S  GLASS   MANUFACTURE. 

to  be  judged  by  looking  through  the  sheet  in  the  ordinary 
way.  It  follows  from  this  fact  that  for  practical  purposes, 
where  the  light  always  traverses  one  thickness  of  the  glass 
only,  a  slight  difference  of  colour  should  be  regarded  as  a 
very  minor  consideration,  at  all  events  as  compared  with 
freedom  from  other  defects. 

The  gathering  process  in  its  turn  is  responsible  for 
further  defects  of  sheet-glass.  Some  of  these,  such  as 
defects  arising  from  the  use  of  a  dirty  pipe,  are  never 
allowed  to  pass  beyond  the  sorting-room,  and  are  therefore 
of  no  interest  to  the  user  of  glass.  Of  those  whose  traces 
are  seen  in  the  glass  that  passes  into  use,  "  blisters  "  and 
"  string  "  are  the  most  important.  "  Blisters  "  are  some- 
what larger,  flat  air-bells,  arising  from  the  inclusion  of 
air  between  successive  layers  of  the  gathering.  "  String  " 
is  a  very  common  defect  in  all  sheet-glass.  To  some 
extent  it  may  arise  from  want  of  homogeneity  in  the  glass 
itself.  If  this  consists  of  layers  of  different  densities  and 
viscosities,  the  gatherer  will  take  these  up  on  his  gathering, 
and  ultimately  they  will  form  thickened  ridges  of  glass 
running  around  the  cylinders  and  across  the  sheets.  Such 
strise,  due  to  want  of  homogeneity  in  the  glass,  are  much 
more  common  in  flint  glass  than  in  the  soda-lime  glasses 
used  for  sheet  manufacture,  but  are  not  unknown  in  the  latter. 
On  the  other  hand,  even  if  the  glass  be  as  homogeneous  as 
possible,  the  gatherer  can  produce  these  striae  if  he  takes 
up  his  glass  from  a  place  close  to  the  side  of  the  fire-clay 
ring  that  floats  in  the  furnace  in  front  of  his  working 
opening.  Glass  always  acts  chemically  upon  fire-clay, 
gradually  forming  a  layer  of  glass  next  to  the  fire-clay  that 
contains  much  more  alumina  than  the  rest  of  the  contents 


SHEET  AND   CEOWN   GLASS.  169 

of  the  furnace.  Such  a  layer  is  formed  on  the  surface  of 
each  ring  in  a  sheet  tank,  but  if  the  gathering  is  taken 
from  the  centre  of  the  ring,  this  thick  viscous  layer  of 
aluminiferous  glass  remains  undisturbed.  If,  however,  the 
gatherer  brings  his  pipe  too  near  the  side  of  the  ring,  the 
glass  will  draw  some  of  this  different  layer  on  to  the 
gathering,  and  this  glass  will  form  thick  ridges  and  striae 
running  across  the  sheet  in  all  directions.  Another  defect 
for  which  the  gatherer  is  generally  responsible  is  that  of 
variation  of  thickness  within  the  same  sheet.  The  blower, 
however,  can  also  produce  this  defect. 

During  the  blowing  proper,  a  further  series  of  defects 
may  be  introduced,  principally  by  allowing  particles  of 
glass  derived  from  certain  stages  of  the  process  to  fall  upon 
the  hot  glass  of  the  cylinder  and  there  become  attached 
permanently.  More  serious,  and  also  more  frequent,  is  the 
greater  or  less  malformation  of  the  cylinder.  If  the  glass 
as  it  leaves  the  blower  is  of  any  shape  other  than  that  of  a 
true  cylinder,  it  becomes  impossible  to  spread  it  into  a 
truly  flat  sheet  in  the  flattening  kiln.  Sometimes,  in 
practice,  the  "cylinder"  is  wider  at  one  end  than  at  the 
other,  or,  worse  still,  it  is  of  uneven  diameter,  showing 
expanded  and  contracted  areas  alternately.  When  such  a 
cylinder  comes  to  be  spread  out  on  the  slab  it  cannot  be 
flattened  completely,  and  various  hollows  and  hillocks  are 
left,  which  mar  the  flatness  of  the  sheet  and  interfere  with 
the  regular  passage  of  light  through  it  when  in  use. 

Finally,  the  process  of  flattening  is  apt  to  introduce 
defects  of  its  own.  The  most  common  of  these  are 
scratches  arising  from  marks  left  by  the  flattening  tool ; 
indeed,  in  all  sheet  glass  it  is  quite  possible  to  see,  by 


170  GLASS  MANUFACTUEE. 

careful  examination  of  the  surfaces,  upon  which  side  the 
flattening  tool  was  used.  Sheet-glass  thus  has  one  side 
decidedly  brighter  and  better  in  surface  than  the  other,  the 
better  side  being  that  which  rested  upon  the  "lagre"  during 
the  flattening  process.  On  the  other  hand,  if  the  slab  itself 
be  not  quite  perfect,  or  if  any  foreign  body  be  allowed  to 
rest  upon  it,  that  side  of  the  glass  will  be  marked  in  a 
corresponding  manner. 

In  the  account  of  the  manufacture  of  sheet-glass  given 
above,  we  have  outlined  one  typical  form  of  the  process, 
but  nearly  every  stage  is  subject  to  modifications  according 
to  the  practice  and  particular  circumstances  of  each  works. 
We  will  now  describe  one  or  two  special  modifications  that 
are  of  more  general  importance. 

First,  as  regards  the  melting  process,  although  the  tank- 
furnace  has  almost  entirely  superseded  the  pot  furnace  for 
the  production  of  ordinary  sheet-glass,  there  are  still  some 
special  circumstances  under  which  the  pot  furnace  is 
capable  of  holding  its  own.  Thus,  where  for  special 
purposes  it  is  desired  to  produce  a  variety  of  sheet-glass 
which,  as  regards  all  defects  arising  out  of  the  glass  itself, 
and  especially  as  regards  colour,  is  required  to  be  as  perfect 
as  possible,  melting  in  pots  is  found  advantageous,  and  for 
some  very  special  purposes  even  covered  (hooded)  pots  are 
used.  For  such  special  purposes,  too,  sulphate  of  soda  is 
eliminated  from  the  raw  materials  and  carbonate  of  soda 
(soda  ash)  substituted.  For  the  production  of  tinted 
glasses  also,  whether  they  are  tinted  throughout  their  mass, 
or  merely  covered  with  a  thin  layer  of  tinted  glass 
("  flashed "),  manufacture  in  pot  rather  than  tank 
furnaces  is  generally  adopted,  the  exact  nature  and  com- 


SHEET  AND   CEOWN  GLASS.  171 

position  of  the  glass  being  far  better  under  control  in  the 
case  of  pots. 

The  blowing  process  is  also  subject  to  wide  variations  of 
practice.  The  most  important  of  these  variations  concerns 
the  shape  and  dimensions  of  the  cylinders.  In  English 
and  Belgian  works  the  dimensions  of  the  cylinders  are  so 
chosen  that  the  length  of  the  cylinder  constitutes  the 
longest  dimension  of  the  finished  sheet,  the  diameter  of  the 
cylinder  forming  the  shorter  dimension.  In  some  parts  of 
Germany,  however,  the  practice  is  the  reverse  of  this,  the 
cylinders  being  blown  shorter  and  much  wider,  so  that  the 
circumference  of  the  cylinder  constitutes  the  longest  dimen- 
sion of  the  finished  sheet.  It  is,  however,  pretty  generally 
recognised  that  the  latter  method  has  very  serious  dis- 
advantages, although  it  is  claimed  that  somewhat  more 
perfect  glass  can  be  obtained  by  its  means.  For  the  pro- 
duction of  a  special  variety  of  glass,  known  as  "  blown 
plate  glass,"  this  method  of  blowing  short  wide  cylinders 
is  still  adhered  to.  This  is  a  very  pure  form  of  sheet-glass, 
blown  into  thick,  small  sheets  which  are  subsequently 
ground  and  polished  in  the  same  manner  as  plate-glass. 
Here  the  great  thickness  of  glass  required  seems  to  render 
the  blowing  of  long  cylinders  very  difficult,  and  the  other 
form  is  therefore  adopted.  On  the  other  hand,  English 
patent  plate-glass,  which  is  made  by  grinding  and  polishing 
the  best  quality  of  ordinary  sheet-glass,  is  made  from  glass 
blown  into  long  narrow  cylinders  in  the  manner  described 
in  detail  above. 

The  process  of  blowing  described  above  is  capable,  with 
slight  modifications,  of  yielding  glass  with  surfaces  other 
than  the  plain  smooth  face  of  ordinary  sheet-glass.  Thus 


172  GLASS  MANUFACTURE. 

fluted  and  "  muffled  "  glass  are  produced  in  a  very  similar 
manner  to  that  described  above  for  ordinary  sheet,  except 
that  the  fluting  or  the  irregular  surface  markings  which 
constitute  the  peculiarities  of  these  two  varieties  of  glass, 
are  impressed  upon  the  surface  of  the  cylinder  at  an  early 
stage  in  the  process. 

From  the  outline  description  given  above  of  the  usual 
method  of  manufacture  of  sheet-glass,  it  will  readily  be  seen 
that  this  is  a  long,  complicated,  and  laborious  process, 
involving  the  employment  of  much  skilled  labour,  and 
involving  the  production  of  a  relatively  complicated  form, 
viz.,  the  closed  cylinder,  as  a  preliminary  to  the  production 
of  a  very  simple  form,  viz.,  the  flat  sheet.  It  is  therefore 
by  no  means  surprising  to  find  that  a  great  many  inventors 
have  worked  and  are  still  working  at  the  problem  of  a 
direct  mechanical  method  of  producing  flat  glass  possessing 
a  natural  "  fire  polish  "  at  least  equal  to  that  of  ordinary 
sheet-glass.  The  earlier  inventors  have  almost  uniformly 
endeavoured  to  attain  this  object  by  attempting  to  improve 
the  process  of  rolling  glass,  with  a  view  to  obtaining  rolled 
sheets  having  a  satisfactory  surface.  We  have  already 
indicated  why  these  efforts  have  never  met  with  success 
and  what  reasons  there  are  for  believing  that  they  are 
never  likely  to  attain  their  object.  A  totally  different  line 
is  that  taken  by  Sievert,  to  whose  inventions  we  have 
already  referred  in  connection  with  the  mechanical  produc- 
tion of  blawn  articles.  This  inventor  has  endeavoured  to 
utilise  his  process  for  blowing  large  articles  of  glass  for  the 
direct  production  of  sheets  of  flat  glass.  His  method  is  to 
blow,  by  the  steam  process  described  in  another  chapter,  a 
large  cubical  vessel,  having  flat  sides,  the  flatness  of  these 


SHEET  AND   CEOWN  GLASS.  173 

sides  being  ensured  by  blowing  the  vessel  into  or  against 
a  mould  having  flat  sides.  This  flat-sided  vessel  is  ulti- 
mately to  be  cut  up  into  five  large  sheets.  This  process 
also  appears  to  involve  some  of  the  main  difficulties  of 
rolling  as  regards  the  means  of  transferring  the  glass  from 
the  furnace  to  the  plate  of  the  blowing  machine,  and  in 
practice  the  inventor  has  not  yet  succeeded  in  producing 
glass  of  sufficiently  good  surface  for  the  purposes  of  sheet 
glass. 

Another  class  of  processes  entirely  avoid  all  means  of 
transferring  molten  glass  from  the  furnace  to  any  machine, 
by  working  on  glass  direct  from  the  molten  bath  itself. 
Some  of  these  processes  are  in  actual  use  in  America,  and 
others  are  being  experimented  with  in  Europe,  but  their 
complete  technical  and  commercial  success  has  yet  to  be 
proved  ;  there  can,  however,  be  little  doubt  that  they  have 
overcome  the  greatest  of  the  many  difficulties  that  stood  in 
the  way  of  the  mechanical  production  of  sheet-glass,  and 
that  they  are  therefore  destined  very  shortly  to  solve  the 
problem  completely,  in  which  case  they  would,  of  course, 
rapidly  supersede  the  hand  process. 

One  of  the  earliest  of  these  direct  processes  proposed  to 
allow  the  molten  glass  to  flow  out  from  the  furnace,  down- 
ward, through  a  narrow  slit  formed  in  the  side  or  bottom  of 
the  tank.  The  impossibility  of  keeping  such  a  narrow 
orifice  open  and  at  the  same  time  regulating  the  flow  of 
glass  made  this  proposal  impracticable,  although  the  use  of 
drawing  orifices  has  been  revived  in  one  of  the  latest 
processes. 

The  American  process,  which  is  said  to  be  at  work 
under  commercial  conditions,  is  not  entirely  satisfactory 


174  GLASS  MANUFACTUEE. 

in  this  respect — that  it  is  a  mechanical  process  for 
the  production  of  cylinders  and  not  of  flat  sheets,  so 
that  the  subsidiary  processes  of  splitting  and  flattening 
still  remain  to  be  carried  out  as  before.  In  this  process  an 
iron  ring  is  lowered  into  the  bath  of  molten  glass  through 
an  aperture  from  above ;  the  glass  is  allowed  to  adhere  to 
the  ring  which  is  then  slowly  raised  by  mechanical  means, 
drawing  a  cylinder  of  glass  with  it.  If  left  to  itself,  such  a 
cylinder,  owing  to  the  effects  of  surface  tension  in  the  glass, 
would  soon  contract  and  break  off,  but  the  American  inven- 
tion avoids  this  action  by  chilling  each  bit  of  the  cylinder 
as  soon  as  it  is  formed.  This  is  done  by  the  aid  of  air 
blasts  delivered  upon  both  sides  of  the  glass  as  it  emerges 
from  the  bath,  and  it  is  claimed  that  by  this  means  cylinders 
of  any  desired  length  and  diameter  may  be  drawn  direct 
from  the  bath.  The  obviously  great  mechanical  difficulties 
connected  with  these  operations  have  probably  been  over- 
come, but  not  without  sacrificing  much  of  the  simplicity  of 
the  arrangement,  and  the  relative  economy  of  this  process 
as  a  whole,  compared  with  the  hand  process,  has  yet  to 
assert  itself. 

The  inventions  of  Fourcault,  which  are  at  present  being 
developed  on  the  Continent  by  a  syndicate  of  glass  manu- 
facturers, aim  at  a  much  more  direct  process.  Here  also 
the  glass  is  drawn  direct  from  the  molten  bath  by  the  aid 
of  a  drawing-iron  that  is  immersed  in  the  glass  and  then 
slowly  raised,  but  in  this  case  the  piece  immersed  is  simply 
a  straight  bar,  and  the  aim  is  to  draw  out  a  flat  sheet.  In 
this  case  the  tendency,  under  surface  tension,  is  to  contract 
the  sheet  into  a  thread,  and  apparently  the  simple  device 
of  chilling  the  emerging  glass  is  not  adequate  to  prevent 


SHEET  AND   CROWN   GLASS.  175 

this  in  a  satisfactory  manner,  and  subsidiary  devices  have 
been  added.  Those  that  have  been  patented  include  a 
mechanism  of  linked  metal  rods  so  arranged  as  to  be 
immersed  and  drawn  out  of  the  glass  continuously  with  the 
emerging  sheet,  in  such  a  manner  as  to  support  the  vertical 
edges  of  the  glass  and  so  aid  in  resisting  the  tendency  of 
the  glass  to  contract  laterally.  Another  device  consists  in 
the  use  of  a  slit  or  orifice  formed  in  a  large  fire-brick  that 
floats  on  the  surface  of  the  glass.  Through  this  orifice  the 
glass  is  drawn,  of  the  desired  thickness  and  width.  The 
use  of  this  orifice,  however,  interferes  markedly  with  the 
perfection  of  the  product,  and  in  fact  all  the  glass  produced 
in  this  way  shows  quite  plainly  a  set  of  longitudinal  stria- 
tions  due  to  the  inevitable  irregularities  in  the  lips  of  the 
drawing  slot.  Further,  it  appears  to  be  impracticable  to 
draw  thin  glass  in  this  way,  a  thickness  of  from  2J  to  3  milli- 
metres (about  ^  inch)  being  the  least  that  is  practicable,  on 
account  of  the  large  amount  of  breakage  that  occurs  with 
weaker  sheets.  This  process,  in  its  present  stage  of  develop- 
ment, however  promising,  does  not  appear  to  have  solved 
the  problem  of  mechanical  manufacture  of  sheet-glass,  since 
it  is  just  in  the  thinner,  lighter  kinds  of  glass  that  the 
advantages  of  sheet  are  most  pronounced.  On  the  other 
hand,  it  is  quite  possible  that  this  drawing  process,  or  some 
development  arising  from  it,  may  shortly  supplant  the 
casting  process  in  the  production  of  polished  plate-glass, 
although  for  the  largest  sizes  of  this  product  also,  the 
difficulty  and  danger  of  handling  the  weights  involved  may 
prove  a  serious  obstacle. 

Crown  Glass. — Although  this  is  a  branch  of  manufacture 
that  is  nearly  obsolete,  it  deserves  brief  notice  here,  partly 


176  GLASS  MANUFACTUKE. 

because  it  is  still  used  for  the  production  of  special  articles, 
and  also  because  it  illustrates  some  interesting  possibilities 
in  the  use  and  manipulation  of  glass. 

The  process  of  blowing  crown  glass  may  be  briefly 
described  as  that  of  first  blowing  an  approximately 
spherical  hollow  ball,  then  opening  this  at  one  side  and 
expanding  the  glass  into  a  flat  disc  by  the  action  of  centri- 
fugal forces  produced  by  a  rapid  rotation  of  the  glass  in 
front  of  a  large  opening  in  a  special  heating  furnace.  The 
actual  process  involves,  of  course,  the  preliminary  of 
gathering  the  proper  quantity  of  glass,  much  in  the  manner 
already  described  in  connection  with  sheet-glass  manufacture. 
This  gathering  is  then  blown  out  into  a  hollow  spherical 
vessel.  This  vessel  is  now  attached  to  a  subsidiary  iron 
rod  by  means  of  a  small  gathering  of  hot  glass,  applied  at 
the  point  opposite  the  pipe  itself,  the  glass  being  thus,  for 
a  moment,  attached  to  both  the  pipe  and  the  "  pontil  "  or 
"punty"  (as  the  rod  is  called).  The  pipe  is,  however, 
detached  by  -cracking  off  the  neck  of  the  original  glass, 
which  now  remains  attached  to  the  pontil  in  the  shape  of 
an  open  bowl.  This  bowl  is  now  re-heated  very  strongly  in 
front  of  a  special'furnace,  the  open  side  of  the  bowl  being 
presented  to  the  fire.  The  pontil  is  meanwhile  held  in  a 
horizontal  position  and  rotated.  As  the  glass  softens  the 
rotation  spreads  it  out,  until  finally  the  entire  mass  of 
glass  is  formed  into  a  simple  flat  disc  spinning  rapidly 
before  the  mouth  of  the  furnace.  This  flat  disc  or  "  table  " 
of  crown  glass  is  allowed  to  cool  somewhat,  is  detached 
from  the  pontil  by  a  sharp  jerk,  and  is  then  annealed  in  a 
simple  kiln  in  which  the  glass  is  stacked,  sealed  up,  and 
allowed  to  cool  naturally. 


SHEET  AND  CROWN  GLASS.  177 

It  is  obvious  that  by  this  process  no  very  large  sheets  of 
glass  can  be  produced  ;  tables  4  ft.  in  diameter  are  already 
on  the  large  side,  and  these  can  only  be  cut  up  into  much 
smaller  sheets  on  account  of  the  lump  of  glass  by  which 
the  table  was  originally  attached  to  the  pontil,  and  which 
remains  fixed  in  the  centre  of  the  finished  disc.  For  certain 
ornamental  purposes,  where  an  "  antique  "  appearance  is 
desired,  these  bullions  are  valued,  but  for  practical  purposes 
they  interfere  very  seriously  with*  the  use  of  the  glass.  As 
a  matter  of  fact,  even  several  inches  away  from  the  central 
bullion  itself,  crown  glass  is  generally  marked  with  circular 
wavings,  which  render  it  readily  recognisable  in  the  windows 
of  older  buildings,  but  which  decidedly  detract  from  the 
perfection  of  the  glass.  On  the  other  hand,  crown  glass  is 
still  valued  for  certain  purposes,  such  as  microscope  slides 
and  cover  glasses,  where  entire  freedom  from  surface 
markings,  such  as  those  found  in  sheet  glass  as  a  result  of 
the  flattening  operations,  is  desirable.  While,  therefore, 
the  process  has  merely  an  historical  interest  so  far  as 
ordinary  sheet-glass  purposes  are  concerned,  it  is  still  used 
in  special  cases. 


G.M.  N 


CHAPTER  XL 

COLOURED    GLASSES. 

IN  various  chapters  throughout  the  foregoing  portions  of 
this  book  we  have  had  occasion  to  refer  to  the  colour  of 
glass  and  the  causes  affecting  it,  but  these  references  have 
chiefly  been  made  from  the  point  of  view  of  the  production 
of  glasses  as  nearly  colourless  as  possible  under  the  circum- 
stances. While  it  is  obvious  that  for  the  great  majority  of 
the  purposes  for  which  it  is  used  the  absence  of  all  visible 
coloration  is  desirable  or  even  essential  in  the  glass 
employed,  there  are  numerous  other  uses  where  a  definite 
coloration  is  required.  Thus  we  have,  as  industrial 
and  technical  uses  of  coloured  glass,  the  employment  of 
ruby,  green  and  purple  glasses  for  signalling  purposes,  as 
in  the  signal  lamps  of  our  railways,  the  red  tail-lights  of 
motor-cars,  or  even  the  red  or  green  sectors  of  certain 
harbour  lights  and  lighthouses  ;  again,  coloured  glasses, 
ruby,  green,  and  yellow,  are  extensively  employed  in  con- 
nection with  photography.  Eather  less  exacting  in  their 
demands"  upon  the  correctness  of  the  colour  employed  are 
the  architectural  and  ornamental  uses  to  which  coloured 
glass  is  so  extensively  put  in  both  public  and  domestic 
buildings,  while,  finally,  coloured  glass  is  largely  the 
foundation  upon  which  the  stained-glass  worker  builds  up 


COLOURED   GLASSES.  179 

his  artistic  achievements  ;  in  another  direction,  coloured 
glass  is  also  utilised  in  the  production  of  ornamental  articles 
and  of  some  table-ware.  While  it  must  be  admitted  that 
in  a  great  many  cases  the  colour-resources  of  the  glass 
maker  are  hopelessly  misapplied,  yet  in  really  artistic  hands 
few  other  materials  are  capable  of  yielding  results  of  equal 
beauty. 

By  the  "  colour  "  of  a  glass  is  generally  understood  the 
tint  or  colour  which  is  observed  when  it  is  viewed,  in  com- 
paratively thin   slices,    by  transmitted    light ;    the  actual 
colour   is  thus  a  property,  not  so  much  of  the  kind  or 
variety  of  glass  as  of  each  individual  piece,  since  thick 
pieces  out  of  the  same  melting  will  show  a  different  tint 
from   that  seen  in  thinner  pieces.     As  we  have  already 
pointed  out,  such  glasses  as  sheet  or  plate,  which  appear 
practically    colourless  when  viewed  in  the  ordinary  way, 
show  a  very  decided  green  colour  when  viewed  through  a 
considerable  thickness.     In  the  same  way,  a  very  thin  layer 
of  the  glass  known  as  "  flashing  ruby  "  shows  a  brilliant  red 
tint,  but  a  thickness  of  one-sixteenth  of  an  inch  is  sufficient 
to  render  the  glass  practically  opaque,  giving  it  a  black 
appearance  by  both  transmitted  and  reflected  light.     Again, 
cobalt  blue  glass,  when  examined  with  a  spectroscope  in 
thin  layers,  is  found   to  transmit  a  notable  proportion   of 
red  rays,  but  thicker  pieces  entirely  suppress  these  rays. 
These   phenomena   will  be   readily   understood   when  we 
recollect  that  colour  in  a  transparent  medium  arises  from 
the  fact  that  the  medium  has  different  absorbing  powers  for 
light  of  different  colours.     All  transparent  substances,  and 
certainly   glass,    are   only  partially  transparent :    all  light 
waves   passing    through    such   a    substance  are  gradually 

N  2 


180  GLASS  MANUFACTUEE. 

absorbed,  and  the  extent  to  which  they  are  absorbed  differs 
according  to  the  length  of  these  waves.     It  always  happens 
that  for  some  special  wave-lengths  the  substance  has  the 
power  of  absorbing  the  energy  of  the  entering  waves  and 
converting  it  into  heat-vibrations  of  its  own  molecules  or 
atoms.     In  the  most  transparent  and  colourless  glasses  this 
process,  so  far  as  the  waves  of  ordinary  light  are  concerned, 
only  goes  on  to  a  negligibly  slight  extent ;  if,  however,  we 
extend  our  view  beyond  the  range  of  ordinary  visible  light, 
and   consider   the   region   of    shorter   waves   that   lies   in 
the   spectrum   beyond   the   violet,  we   find   that   ordinary 
colourless  glass  becomes  strongly  absorbent ;  thus  to  waves 
of  about  half  the  length  of  those  which  produce  upon  our 
eyes  the  impression  of  yellow  light,  ordinary  glass  is  as 
opaque  as  is  a  piece  of  metal  to  white  light.     In  this  wider 
sense,  then,  we  may  fairly  say  that  all  glasses  are  coloured — 
i.e.,  all  have  a  power  of  selective  absorption  ;  but  in  the 
case  of  those  which  are  nearly  colourless  in  the  ordinary 
sense,  this  absorption  takes  place  only  for  waves  which  are 
either  decidedly  shorter  or  decidedly  longer  than  those  to 
which  our  eyes  are  sensitive.     Those  glasses  which  appear 
coloured  in  the  ordinary  sense,  on  the  other  hand,  owe  this 
property  to  the  fact    that    the    power    of    absorption    for 
light-waves  extends  into  the  region  of  the  visible  spectrum  ; 
thus  a  blue  or  violet  glass  is  practically  opaque  to  red  rays, 
while  a  red  glass  is  opaque  to  blue,  green  or  violet  rays. 
This  statement  may  be  verified  in  a  striking  manner  by 
holding  over  one  another  a  piece  of  deep  blue  or  green 
glass  and  a  similar  piece  of  ruby  glass — the  combination 
will  be  found  to  be  very  nearly  opaque  even  when  each 
glass  by  itself  is  practically  transparent. 


COLOUBED   GLASSES.  181 

The  question  which  now  naturally  presents  itself  to  us  is, 
what  is  the  essential  difference  between,  for  instance,  a 
piece  of  red  glass  and  a  piece  of  "  white  "  glass  that  confers 
upon  the  former  the  power  of  absorbing  blue  light  ?  A 
perfectly  complete  and  satisfactory  answer  to  this  question 
is  not,  in  the  writer's  opinion,  available  in  the  present  state 
of  our  knowledge,  but  to  a  certain  extent  the  difference 
between  the  two  kinds  of  glass  can  be  explained.  The 
difference  is  produced,  in  the  /first  instance  by  introducing 
into  the  colourless  glass  some  additional  chemical  element 
or  elements,  the  substances  in  question  being  generally 
known  as  "  colouring  oxides,"  although  they  are  by  no 
means  always  introduced  in  the  form  of  oxides,  and  are 
frequently  present  in  the  glass  in  entirely  different  forms. 
To  a  certain  extent  the  colour  of  the  glass  may  be  ascribed 
to  a  definite  "  colouring  "  property  of  the  chemical  elements 
concerned ;  thus  most  of  the  chemical  compounds  of  such 
elements  as  nickel,  cobalt,  iron,  manganese  and  copper  are 
more  or  less  deeply  coloured  substances,  and  it  would  seem 
as  if  the  atoms  or  "  ions  "  of  these  elements  had  the  specific 
power  of  absorbing  certain  varieties  of  light-waves  while 
not  materially  affecting  others.  But  this  specific  "  colour- 
ing" property  is  not  so  easily  explained  when  we  recollect 
that  the  colours  of  iron  compounds,  for  example,  may  be 
green  or  red  according  to  the  state  of  combination  in  which 
that  element  is  present,  and  that  iron  has  also  the  power  of 
imparting  either  a  green  or  a  yellow  colour  to  glass  accord- 
ing to  circumstances.  The  detailed  discussion  of  these 
questions,  however,  lies  outside  our  present  scope,  and  we 
must  confine  ourselves  to  the  broad  statement  that  colour- 
ing substance  in  glass  may  be  roughly  divided  into  two 


182  GLASS  MANUFACTURE. 

kinds  or  groups  ;  the  first  and  probably  the  largest  group 
are  those  bodies  which  occur  in  glass  in  true  solution,  the 
element  itself  being  present  in  the  combined  state  as  a 
silicate  or  other  such  compound  (borate,  phosphate,  etc.) 
which  is  soluble  in  the  glass.  In  this  class,  the  colouring 
effect  upon  the  glass  is  specifically  that  of  the  element 
introduced,  and  is  brought  about  in  the  same  way  as  the 
colouring  of  water  when  a  coloured  salt — such  as  copper 
sulphate — is  dissolved  in  it.  The  second  class  of  colouring 
substances,  however,  behave  in  a  different  manner;  they 
are  probably  present  in  the  glass  in  a  state  of  extremely 
fine  division,  and  held  not  in  true  solution,  but  really  in  a 
sort  of  mechanical  suspension  that  approximates  to  the 
condition  of  what  is  known  as  a  "  colloidal  solution."  The 
point  which  is  known  beyond  doubt,  thanks  to  the  researches 
of  Siedentopf  and  Szigmondi  on  ultra-microscopical  particles, 
is  that  in  certain  coloured  glasses,  of  which  ruby  glass  is 
the  best  example,  the  colouring  substance,  be  it  gold  or 
cuprous  oxide,  is  present  in  the  form  of  minute  but  by  no 
means  atomic  or  molecular  particles  suspended  in  the  glass. 
The  presence  of  these  particles  has  been  made  optically 
evident,  although  it  can  hardly  be  said  that  they  have  been 
rendered  visible,  and  it  is  at  all  events  probable  that  these 
suspended  particles  act  each  as  a  whole  in  absorbing  the  light- 
waves characteristic  of  the  colour  which  they  produce  in 
glass.  This  being  the  case,  it  is  easy  to  understand  how 
readily  the  colour  of  such  glasses  is  altered  or  spoilt  by 
manipulations  which  involve  heating  and  cooling  at  different 
rates — too  rapid  a  rate  of  cooling  producing  a  different 
grouping  of  the  minute  particles,  altering  their  size  or 
shape,  or  even  obliterating  them  entirely  by  allowing  the 


COLOUEED   GLASSES.  183 

element  in  question  to  go  into  or  to  remain  in  solution  in 
the  glass. 

While  it  would  be  entirely  foreign  to  the  purpose  of  this 
volume  to  give  in  this  place  a  series  of  recipes  for  the 
production  of  various  kinds  of  coloured  glass,  it  will  be 
desirable  to  state  in  general  terms  the  colours  or  range  of 
colours  which  can  be  produced  in  various  kinds  of  glass  by 
the  introduction  of  those  chemical  elements  which  are 
ordinarily  used  in  this  way.  In  general  terms  it  may  be 
said  that  the  lighter  elements  do  not  as  a  rule  tend  to  the 
production  of  coloured  glasses,  while  the  heavier  elements, 
so  far  as  they  can  be  retained  in  the  glass  in  either  solution 
or  suspension,  tend  to  produce  an  intense  colouring  effect. 
The  element  lead  appears  to  form  a  striking  exception  to 
this  rule,  but  this  is  due  to  the  fact  that  while  the  silicates 
of  most  of  the  other  heavy  elements  are  more  or  less 
unstable,  the  silicate  of  lead  is  very  stable,  and  can  only  be 
decomposed  by  the  action  of  reducing  agents.  When  lead 
silicates  are  decomposed  in  this  way,  however,  the  resulting 
glass  immediately  receives  an  exceedingly  deep  colour, 
being  turned  a  deep  opaque  black,  although  in  very  thin 
layers  the  colour  is  decidedly  brown.  On  the  other  hand, 
glasses  very  rich  in  lead  are  always  decidedly  yellow  in 
colour,  and  it  has  been  shown  that  this  coloration  is  due 
to  the  natural  colour  of  lead  silicates  and  not  to  the  presence 
of  impurities.  What  has  just  been  said  of  lead  applies, 
with  only  very  slight  modification,  also  to  the  rare  metal 
thallium  and  its  compounds,  which  have  been  introduced 
into  glass  for  special  purposes.  Leaving  these  two  excep- 
tional bodies  on  one  side,  we  now  pass  to  a  consideration  of 
the  elements  in  the  order  of  their  chemical  grouping.  The 


184  GLASS   MANUFACTURE. 

rare  elements  will  not  be  considered  except  in  certain  cases 
where  their  presence  in  traces  is  liable  to  affect  results 
attained  in  practice. 

The  Alkali  Metals,  sodium,  potassium,  lithium,  etc.,  and 
their  compounds,  have  no  specific  colouring  effect,  although 
the  presence  of  soda  or  of  potash  in  a  glass  affects  the 
colours  produced  by  such  substances  as  manganese,  nickel, 
selenium,  etc. 

Copper,  as  would  be  anticipated  from  the  deep  colour  of 
most  of  its  compounds,  produces  powerful  colouring  effects 
on  glass.  Cupric  silicates  produce  intense  green,  to  greenish- 
blue  tints.  Copper,  either  as  metal  or  oxide,  added  to  glass 
in  the  ordinary  way,  always  produces  the  green  colour  ;  but 
when  the  full  oxidation  of  the  copper  is  prevented  by  the 
presence  of  a  reducing  body,  and  the  glass  is  cooled  slowly, 
or  is  exposed  to  repeated  heating  followed  by  slow  cooling, 
an  intense  ruby  coloration  is  produced.  In  practice  this 
colour  is  produced  by  introducing  tin  as  well  as  copper  into 
the  mixture,  and  so  regulating  the  conditions  of  melting  as 
to  favour  reduction  rather  than  oxidation  of  the  copper. 
Under  these  circumstances  the  copper  is  left  in  the  glass  in 
a  finely  divided  and  evenly  suspended  state ;  if  exactly  the 
right  state  of  division  and  suspension  is  arrived  at,  a 
beautiful  red  tint  is  the  result,  although  the  coloration  of 
the  glass  is  so  intense  that  it  can  only  be  employed  in  very 
thin  sheets,  being  "  flashed  "  upon  the  surface  of  colourless 
glass  to  give  it  the  necessary  strength  and  thickness  for 
practical  use.  It  is  further  very  easy  to  slightly  alter  the 
arrangement  of  the  copper  in  the  glass,  with  the  result  of 
producing  an  opaque,  streaky  substance  resembling  sealing- 
wax  in  colour  and  appearance,  this  product  being,  of  course, 


COLOURED   GLASSES.  185 

useless  from  the  glass-maker's  point  of  view.  Finally,  by 
exceedingly  slow  cooling,  and  under  other  favouring  condi- 
tions which  are  not  really  understood,  the  particles  of 
suspended  colouring-material — be  it  metallic  copper  or 
cuprous  oxide — grow  in  size  and  attain  visible  dimensions, 
appearing  as  minute  shimmering  flakes,  thus  producing  the 
beautiful  substance  known  as  "  aventurine." 

Silver  is  never  introduced  into  glass  mixtures,  the  reason 
being  that  it  is  so  readily  reduced  to  the  metallic  state 
from  all  its  compounds  that  it  cannot  be  retained  in  the 
glass  except  in  a  finely-divided  form,  causing  the  glass  to 
assume  a  black,  metallic  appearance  resembling  the  stains 
produced  by  the  reduction  of  lead  in  flint  glasses.  On  the 
other  hand,  silver  yields  a  beautiful  yellow  colour  when 
applied  to  glass  as  a  surface  stain,  and  it  is  widely  used  for 
that  purpose. 

Gold  is  introduced  into  glass  for  the  production  of  brilliant 
ruby  tints ;  its  behaviour  is  very  similar  to  that  of  copper, 
except  that  the  noble  metal  has  a  great  tendency  to  return 
to  the  metallic  state  without  the  aid  of  reducing  agents. 
No  addition  of  tin  is  therefore  required,  but  the  rate  of 
cooling,  etc.,  must  be  properly  regulated,  since  rapidly 
cooled  glass  containing  gold  shows  no  special  colour,  the 
rich  ruby  tint  being  only  developed  when  the  glass  is 
re-heated  and  cooled  slowly.  The  colouring  effect  of  gold 
is  undoubtedly  more  regular  and  uniform  than  that  of 
copper,  and  it  is  accordingly  possible  to  obtain  much  lighter 
shades  of  red  with  the  aid  of  the  noble  metal.  "  Gold  ruby  " 
can  therefore  be  obtained  of  a  tint  light  enough  to  be  used 
in  sheets  of  ordinary  thickness,  and  the  process  of  "  flashing  " 
is  not  essential. 


186  GLASS  MANUFACTURE. 

The  elements  of  the  second  group,  such  as  magnesium, 
calcium,  strontium,  barium,  zinc  and  cadmium,  exert  no 
strong  specific  colouring  action  on  glass,  with  perhaps  the 
exception  of  cadmium,  and  that  element  only  does  so  to 
any  considerable  extent  in  combination  with  sulphur, 
sulphide  of  cadmium  having  the  power  of  producing  rich 
yellow  colours  in  glass.  The  sulphur  compounds  of  barium 
also  readily  produce  deep  green  and  yellow  colours,  and  the 
formation  of  these  tints  is,  indeed,  very  difficult  to  avoid  in 
the  case  of  glasses  containing  much  barium.  A  colouring 
effect  has  sometimes  been  ascribed  to  zinc,  but  this  is  not 
in  accordance  with  facts. 

Of  the  elements  of  the  third  group,  only  boron  and 
aluminium  are  ever  found  in  glass  in  any  notable  quantity. 
Boron  is  present  in  the  form  of  boric  acid  or  borates,  and  as 
such  produces  no  colouring  effect,  nor  does  there  seem  to  be 
any  tendency  for  the  separation  of  free  bor.on.  The  com- 
pounds of  aluminium  also  possess  no  colouring  effect, 
although  certain  compounds  of  this  element  are  utilised  for 
imparting  a  white  opacity  to  glass  for  certain  purposes — 
such  glass  being  known  as  "qgal." 

The  elements  of  the  fourth  group  are  of  greater  importance 
in  connection  with  glass.  Carbon  is  capable  of  exerting 
powerful  colouring  effects  when  introduced  into  glass. 
These  effects  are  of  two  kinds,  viz.,  indirect  in  consequence 
of  the  reducing  action  of  carbon  on  other  substances 
present,  and  direct  from  the  presence  of  finely-divided 
carbon  or  carbides  in  the  glass.  The  latter  are  similar  in 
kind  to  those  produced  by  the  presence  of  other  finely- 
divided  elementary  bodies  (copper,  gold,  lead,  etc.)  except 
that  the  lightness  of  the  carbon  particles  tends  to  the 


COLOITEED  GLASSES.  187 

production  of  yellow  and  brown  colours  rather  than  of  red 
and  black,  while  the  chemical  nature  of  carbon  renders  the 
glass  in  which  it  is  suspended  indifferent  to  rapid  cooling, 
so  far  as  the  carbon  tint  is  concerned.  The  indirect  effects 
of  carbon,  in  reducing  other  substances  that  may  be  present 
in  the  glass,  become  evident  with  much  smaller  proportions 
of  carbon  than  are  required  to  produce  visible  direct  effects. 
As  we  have  seen  above,  carbon,  in  the  form  of  coke,  char- 
coal or  anthracite  coal,  is  regularly  introduced,  as  a  reducing 
medium,  into  glass  mixtures  containing  sulphate  of  soda. 
If  even  a  slight  excess  of  carbon  be  used  for  this  purpose, 
the  formation  of  sulphides  and  poly-sulphides  of  sodium 
and  of  calcium  results,  and  these  bodies,  like  all  sulphides, 
impart  a  greenish-yellow  tint  to  the  glass,  at  the  same 
time  bringing  other  undesirable  results  in  their  train. 

Silicon,  in  the  form  of  silicic  acid  and  its  compounds,  is  a 
fundamental  constituent  of  all  varieties  of  glass,  and  in  this 
form  is  in  no  sense  a  colouring  substance ;  on  the  other 
hand,  there  is  no  doubt  that  under  some  conditions  silicon 
may  be  reduced  to  the  metallic  state  at  temperatures  which 
normally  occur  in  glass-furnaces,  and  it  is  practically 
certain,  that  if  present  in  glass  in  this  condition,  silicon 
would  colour  the  glass.  It  is  just  possible  that  some  of  the 
colouring  effects  produced  in  ordinary  glass  by  powerful 
reducing  agents,  such  as  carbon,  either  in  the  solid  form  or 
as  a  constituent  of  furnace  gases,  may  be  due  to  the  reduction 
of  silicon  in  the  glass. 

Tin  by  itself  does  not  appear  to  have  any  colouring  effect 
upon  glass,  except  that  its  oxide,  in  a  finely  suspended 
state,  produces  opalescence  and,  in  large  quantities,  white 
opacity.  Tin,  however,  is  used  in  conjunction  with  copper 


188  GLASS  MANUFACTURE. 

in  the  production  of  copper-ruby,  to  which  reference  has 
already  been  made. 

Lead  and  Thallium  have  already  been  dealt  with,  and  it 
only  remains  to  add  that  their  presence  in  the  glass, 
although  not  in  itself  producing  any  intense  colouring 
action,  increases  the  colouring  effects  of  other  substances. 
This  is  probably  merely  a  particular  case  of  the  fact  that 
dense  glasses,  of  high  refractive  index,  are  more  sensitive 
to  colouring  agencies  than  the  lighter  glasses  of  low 
refractive  index ;  this  applies  to  barium  as  well  as  to  lead 
and  thallium  glasses. 

Phosphorus  occurs  in  some  few  glasses  in  the  form  of 
phosphoric  acid,  and  this  substance,  as  such,  has  no  colouring 
effect.  Calcium  phosphate,  however,  is  sometimes  added  to 
glasses  for  the  purpose  of  producing  opalescence.  Its 
action  in  this  respect  is  probably  similar  to  that  of  tin 
oxide  and  aluminium  fluoride,  these  substances  all  remaining 
undissolved  in  the  glass  in  the  form  of  minute  particles  in 
a  finely  divided  and  suspended  state. 

Arsenic  does  not  exert  a  colouring  effect  on  glass,  and 
owing  to  its  volatile  nature  it  can  only  be  retained  in  glass 
in  small  quantities  and  under  special  conditions.  A 
"  decolourising  "  action  is  sometimes  ascribed  to  arsenic, 
but  if  this  action  really  exists  it  can  only  be  ascribed  to  the 
fact  that  arsenic  compounds  are  capable  of  acting  as  carriers 
of  oxygen,  and  their  presence  thus  tends  to  facilitate  the 
oxidation  of  impurities  contained  in  the  glass.  A  further 
reference  to  this  subject  will  be  found  below  in  reference  to 
the  compounds  of  manganese. 

Antimony,  although  frequently  added  to  special  glass 
mixtures,  does  not  appear  to  produce  any  very  power- 


COLOUEED   GLASSES.  189 

ful  effects,  except  possibly  in  the  direction  of  producing 
white  opacity  if  present  in  large  proportions.  The  sulphide 
of  antimony,  however,  exerts  a  colouring  influence,  although 
its  volatile  and  unstable  character  renders  the  effects 
uncertain. 

Vanadium,  owing  to  its  rarity,  is  probably  never  added 
to  glass  mixtures  for  colouring  purposes,  although  it  is 
capable  of  producing  vivid  yellow  and  greenish  tints  when 
present  even  in  minute  proportions.  On  the  other  hand, 
vanadium  occurs  in  small  proportions  in  a  number  of  fire- 
clays, including  some  of  those  of  the  Stourbridge  district, 
and  glass  melted  in  pots  containing  this  element  is  liable 
to  have  its  colour  spoilt  by  taking  up  the  vanadium  from 
the  clay. 

Sulphur  is  an  element  whose  presence  in  various  forms 
is  liable  to  affect  the  colour  of  glass  in  a  variety  of  ways. 
The  colouring  effects  of  sodium-,  calcium-,  cadmium-,  and 
antimony -sulphides  have  already  been  referred  to.  Sulphur 
probably  never  exists  in  glass  in  the  uncombined  state 
at  all,  but  sulphur  and  its  oxides,  which  are  often 
contained  in  furnace  gases,  sometimes  exert  a  very  marked 
action  upon  hot  glass.  The  presence  of  sulphur  gases 
in  the  atmospheres  of  blowing-holes  and  annealing  kilns  is 
liable  to  produce  in  the  glass  a  peculiar  yellowish  milkiness 
which  penetrates  for  a  considerable  depth  into  the  mass  of 
the  glass  and  cannot  be  removed  by  subsequent  treatment. 
Glass  vessels,  particularly  if  made  of  glass  produced  from 
raw  materials  among  which  salt-cake  has  figured,  are  also 
affected  by  contact  with  fused  sulphur  or  its  vapour,  the 
effect  being  a  gradual  disintegration  of  the  glass.  The 
precise  mechanism  of  these  actions  is  not  known  at  present, 


190  GLASS  MANUFACTUKE. 

but  they  probably  consist  in  the  formation  of  sulphur  com- 
pounds within  the  glass,  possibly  giving  rise  to  an  evolution 
of  minute  bubbles  of  gas. 

Selenium,  which  is  chemically  so  closely  related  to 
sulphur,  is  a  relatively  rare  element,  which  is,  however, 
finding  some  use  in  glass-manufacture  as  a  colouring  and  a 
decolouring  agent.  The  introduction  of  selenium  or  of  its 
compounds  under  suitable  conditions  into  a  glass  mixture 
produces  or  tends  to  produce  a  peculiar  yellowish-pink 
coloration,  the  intensity  of  the  colour  produced  being 
dependent  upon  the  chemical  nature  of  the  glass  as  a  whole 
and,  of  course,  upon  the  amount  of  selenium  left  in  the 
glass  at  the  end  of  the  melting  process,  this  latter  in  turn 
depending  upon  the  duration  and  temperature  of  the 
process  in  question.  The  pini  colour  of  selenium  glass  is 
best  developed  in  those  containing  barium  as  a  base,  but  it 
is  also  developed  in  lead  glasses,  while  soda-lime  glasses  do 
not  show  the  colour  so  well.  As  a  "  decolouriser  "  the 
action  of  selenium  is  entirely  that  of  producing  a  comple- 
mentary colour  which  is  intended  to  "  cover"  the  green  or 
blue  tint  of  the  glass  ;  where  the  depth  of  the  tint  to  be 
"covered"  is  small,  selenium  can  be  used  very  successfully 
in  this  way,  although  it  is  a  relatively  costly  substance  for 
such  a  purpose.  No  oxidising  or  "cleansing"  action  can 
be  ascribed  to  selenium  or  its  compounds. 

Chromium  is  one  of  the  most  intensely  active  colouring 
substances  that  are  available  for  the  glass-maker,  and  it  is 
accordingly  used  very  extensively.  It  has  the  advantage  of 
relative  cheapness,  and  can  be  conveniently  obtained  and 
introduced  into  glass  in  the  form  of  pure  compounds  whose 
colouring  effect  can  be  accurately  anticipated  ;  the  colours 


COLOURED  GLASSES.  191 

produced  by  the  aid  of  chromium  have  the  further  advantage 
of  being  very  constant  in  character,  being  little  affected  by 
oxidising  or  reducing  conditions,  and  only  very  slightly  by 
the  length  or  temperature  of  the  melting  process.  The 
rate  of  cooling,  in  fact,  appears  to  be  the  only  factor  that 
materially  affects  the  colours  produced  by  compounds  of 
chromium.  The  colours  produced  by  chromium  alone  are 
various  depths  of  a  bright  green,  the  depth  varying,  of 
course,  with  the  proportion  of  chromium  that  is  present  in 
the  glass  and  with  the  purity  of  the  glass  itself.  Very 
frequently,  chromium  is  used  in  conjunction  with  either 
iron  or  copper  to  produce  various  tints  of  "cold  blue" 
and  "  celadon  green  "  respectively.  This  element  is  most 
usually  introduced  into  the  glass  mixture  in  the  form  of 
potassium  bichromate  ;  although  other  compounds  might 
be  employed,  this  substance  presents  several  advantages  to 
the  glass  maker.  In  the  first  place,  since  the  colouring 
effect  of  chromium  is  very  intense,  it  must  be  used  in  very 
small  quantities,  and  if  chromic  oxide  itself  were  used,  the 
weighing  would  have  to  be  carried  out  with  extreme 
care ;  potassium  bichromate,  however,  contains  a  much 
smaller  proportion  of  the  effective  colouring  substance,  so 
that  much  larger  weights  can  be  employed,  and  the  accuracy 
of  weighing  required  is  proportionately  reduced.  A  further 
consideration  arises  from  the  fact  that  chromic  oxide  is 
itself  an  extremely  refractory  body,  and  is  therefore  com- 
paratively difficult  to  incorporate  with  glass,  while  its 
presence  tends  to  make  the  glass  itself  more  viscid  and 
refractory;  the  simultaneous  introduction  of  the  alkali, 
as  provided  by  the  use  of  the  bichromate,  is  thus  an 
advantage  in  restoring  the  fluidity  and  softness  of  the  glass 


192  GLASS  MANUFACTURE. 

when  finished,  while  also  facilitating  the  solution  of  the 
chromium  in  the  glass  during  the  fusion  process ;  this 
process  of  solution,  however,  takes  some  time,  chromium 
glasses  being  liable  to  appear  patchy  if  insufficient  time  is 
given  to  the  "founding." 

Uranium  is  one  of  the  rarer  and  more  costly  elements, 
but  is  nevertheless  used  in  glass-making  for  special  purposes 
on  account  of  the  very  beautiful  fluorescent  yellow  colour 
which  it  imparts  when  added  in  small  proportions.  This 
yellow  is  quite  characteristic  and  unmistakable,  so  that 
none  of  the  other  varieties  of  yellow  glass  can  ever  be  used 
as  a  substitute  for  uranium  glass,  but  the  great  cost  of  the 
latter  prevents  its  extended  use.  Uranium  is  usually 
introduced  into  glass  mixtures  in  the  form  of  a  chemical 
compound,  such  as  uranyl-acetate  or  uranyl-nitrate,  both 
these  substances  being  obtainable  in  the  form  of  small, 
intensely  bright  yellow  crystals. 

Fluorine  occurs  in  a  number  of  glasses  in  the  form  of 
dissolved  or  suspended  fluorides,  principally  fluoride  of 
aluminium.  The  element  is  not  essentially  a  colouring 
substance,  and  is  only  mentioned  here  because  the  fluoride 
named  is  the  most  frequently  used  means  of  producing 
"opal"  glass.  The  fluoride  is  most  frequently  introduced 
into  the  glass  mixtures  as  calcium  fluoride,  used  in 
conjunction  with  felspar,  or  as  cryolite,  a  natural  mineral 
which  consists  of  a  double  fluoride  of  sodium  and 
aluminium. 

Manganese  is  one  of  the  most  important  colouring 
elements  used  by  the  glass-maker.  When  introduced  into 
glass  in  the  absence  of  other  colouring  ingredients, 
compounds  of  manganese  produce  a  range  of  colours  lying 


COLOURED  GLASSES.  193 

in  the  region  of  pinkish-purple  to  violet,  according  to  the 
chemical  nature  of  the  glass.  The  exact  colour  produced 
varies  according  as  the  glass  has  lead,  lime  or  harium  as 
its  base,  and  it  also  depends  upon  the  presence  of  soda  or 
potash  as  the  alkaline  constituent.  The  nature  and 
intensity  of  the  colour,  however,  which  the  addition  of  a 
given  percentage  of  manganese  will  produce  depends  upon 
other  factors  besides  the  chemical  composition  of  the  bases 
used  in  the  mixture.  The  heat  and  duration  of  the 
"  found  "  and  the  reducing  or  oxidising  conditions  of  the 
furnace  in  which  it  has  been  carried  on  very  materially 
affect  the  result.  Thus,  a  glass  having  a  slight  tinge  of 
pink  or  purple  derived  from  manganese  can  be  rendered 
entirely  colourless  by  the  action  of  reducing  gases  or  by  intro- 
ducing into  the  glass  a  reducing  substance,  such  as  a  piece  of 
wood.  It  will  thus  be  seen  that  while  manganese  is  a  most 
useful  element  for  the  glass-maker,  its  employment  requires 
much  skill  and  care,  and  generally  involves  some  troublesome 
manipulations  before  the  desired  result  is  attained. 

In  practice,  manganese  is  most  frequently  used  with 
other  colouring  ingredients  for  the  production  of  what  may 
be  called  "  compound "  colours,  the  function  of  the 
manganese  being  to  provide  the  "  warm  "  element,  i.e.,  the 
pink  or  purple  component,  required.  One  of  the  most  im- 
portant uses  of  manganese  coming  under  this  head  is  its  use 
as  a  "  decolouriser."  By  a  "  decolouriser  "  the  glass-maker 
understands  a  substance  which  can  be  used  to  improve  the 
colour  of  a  glass  which,  from  the  nature  of  its  raw  materials 
and  conditions  of  melting;  would  have  a  greener  colour  than 
is  thought  desirable  for  the  product  in  question.  It  may 
be  said  at  once  that  the  most  perfect  and  satisfactory 

G.M.  o 


194  GLASS  MANUFACTUBE. 

method  of  obtaining  the  better  colour  required  is  to  adopt 
the  use  of  purer  raw  materials  and  methods  of  melting  less 
liable  to  lead  to  contamination  of  the  glass.  On  the  other 
hand,  this  radical  course  is  often  impossible  on  the  ground 
of  expense,  and  the  less  satisfactory  course  must  be  adopted 
of  covering  one  undesirable  colour  by  another  complementary 
colour  which  would,  in  itself,  be  equally  undesirable.  The 
rationale  of  this  procedure  depends  upon  the  fact  that  a 
slight  amount  of  absorption  of  light  is  not  readily  detected 
by  the  human  eye  if  it  be  uniformly  or  nearly  uniformly 
distributed  over  the  whole  range  of  the  visible  spectrum, 
i.e.,  if  the  colour  of  the  resulting  light  is  nearly  neutral, 
while  an  equally  slight  absorption  in  one  region  of  the 
spectrum,  while  actually  allowing  more  light  to  pass  through 
the  glass,  is  at  once  detected  by  the  eye  owing  to  the  colour  of 
the  transmitted  light.  Now  it  has  been  found  that  the  colour 
produced  in  glass  by  the  addition  of  very  small  proportions 
of  manganese  is  approximately  complementary  to  the 
greenish-blue  tinge  of  the  less  pure  varieties  of  ordinary 
glass ;  the  addition  of  manganese  in  suitable  proportions  to 
such  glass  therefore  results  in  the  production  of  a  glass 
which  transmits  light  of  approximately  neutral,  usually 
slightly  yellow,  colour,  the  increased  total  absorption  only 
becoming  noticeable  in  large  pieces.  This  ''covering"  of 
the  greenish  tinge  is  generally  most  completely  successful 
in  the  case  of  soda-flint  glasses,  but  the  method  is  also  used 
to  a  certain  extent  in  the  case  of  the  soda-lime  glasses  used 
for  sheet  and  plate-glass  manufacture.  Manganese  added 
to  glass  for  this  purpose  is  generally  introduced  into  the 
mixture  in  the  form  of  the  powdered  black  oxide  (manganese 
dioxide),  which  is  available  as  a  natural  ore  in  a  condition 


COLOURED  GLASSES.  195 

of  sufficient  purity.  Added  in  this  form,  the  manganese 
compound  exerts  a  double  action,  the  decomposition  of  the 
dioxide  resulting  in  the  liberation  of  oxygen  within  the 
mass  of  melting  glass,  and  this  oxygen  itself  exerts  a 
favourable  influence  on  the  resulting  colour  of  the  glass, 
since  it  removes  organic  materials  whose  subsequent  reducing 
action  would  be  deleterious,  and  it  also  converts  all  iron 
compounds  present  into  the  more  highly-oxidised  (ferric) 
state  in  which  their  colouring  effects  are  less  intense.  The 
actual  colouring  effect  of  the  manganese  itself  is,  of  course, 
afterwards  developed,  and  produces  the  effects  discussed 
above. 

The  "covering"  of  the  greenish  tints  due  to  iron  and 
other  compounds  is  only  possible  when  these  are  present 
in  very  small  proportions.  When  larger  quantities  of 
these  substances  have  been  introduced  into  the  glass  the 
addition  of  manganese  modifies  the  resulting  colour,  but  is 
no  longer  able  to  neutralise  it.  A  very  large  range  of 
colours  can  be  obtained  by  using  various  proportions  of 
iron  and  manganese,  the  best-known  of  these  being  the 
warm  brown  tint  known  as  "hock-bottle,"  while  all  shades 
between  this  and  the  bright  green  of  iron  and  the  purple  of 
manganese  can  be  obtained  by  suitable  mixtures.  What 
has  been  said  above  as  to  the  sensitiveness  of  manganese 
colours  applies  with  even  greater  force  to  these  mixed  tints, 
since  here  both  the  iron  and  the  manganese  compounds 
are  liable  to  undergo  changes  of  oxidation.  Copper- 
manganese  and  chromium-manganese  colours  are  also 
used,  as  indeed  almost  any  number  of  colouring  ingredients 
may  be  simultaneously  introduced  into  a  glass  mixture, 
the  resulting  colour  being,  as  a  rule,  purely  additive. 

o  2 


19H  GLASS  MANUFACTURE. 

Iron  is  so  widely  distributed  among  the  materials  of  the 
earth's  crust  that  it  is  exceedingly  difficult  to  exclude  it 
entirely  from  any  kind  of  glass,  although  the  purest 
varieties  of  glass  contain  the  merest  traces  of  this  element. 
Cheaper  varieties  of  glass,  however,  always  contain  iron 
in  measurable  quantity,  while  the  cheapest  kinds  of 
glass  contain  considerable  proportions  of  this  element. 
The  colouring  effects  of  iron  have  already  been  alluded  to 
at  various  points  in  the  earlier  chapters  as  well  as  in  the 
section  on  manganese  just  preceding.  Little  further 
remains  to  be  said  here.  Just  as  the  less  highly  oxidised 
compounds  of  iron — i.e.,  the  "  ferrous  "  compounds — always 
show  a  decided  green  tint,  so  glasses  containing  iron  when 
melted  under  the  usually  prevalent  reducing  conditions  of 
a  glass-making  furnace,  show  a  decided  green  tint  whose 
depth  depends  upon  the  amount  of  iron  present,  provided 
no  manganese  or  other  "  decolouriser  "  has  been  introduced. 
"  Ferrous  ;'  compounds  are,  however,  readily  converted 
into  the  more  highly  oxidised  or  "  ferric  "  state  by  the 
action  of  oxidising  agents,  and  this  change  can  also  be 
brought  about  in  molten  glass  by  the  action  of  such  sub- 
stances  as  nitrates  or  other  sources  of  oxygen.  The  ferric 
compounds,  however,  show  characteristic  yellow  tints  which 
are  much  less  intense  and  vivid  than  the  corresponding 
green  colours  of  the  "  ferrous  "  series,  and  a  similar  result 
is  brought  about  by  the  oxidation  of  iron  compounds  con- 
tained in  glass  ;  hence  the  "washing  "  or  cleansing  effects 
ascribed  to  oxidising  agents  introduced  in  the  fusion  of 
glass.  It  should,  however,  be  borne  in  mind  that  the 
oxidation  of  other  substances  besides  iron  compounds,  viz., 
organic  matter,  carbon  and  sulphur  compounds,  may,  and 


COLOURED   GLASSES.  197 

probably  does,  play  a  most  important  part  in  this  process 
in  the  case  of  most  varieties  of  glass. 

Nickel  exerts  a  powerful  colouring  influence  on  glass,  in 
accordance  with  the  fact  that  most  of  the  other  compounds 
of  this  element  are  also  deeply  coloured.  The  exact  colour 
produced  in  glass  depends  upon  the  nature  of  the  glass 
and  on  the  condition  of  oxidation  in  which  the  nickel  is 
present.  The  colours,  however,  are  usually  of  a  greenish- 
brown  tint,  although  brighter  colours  can  be  produced  by 
nickel  under  special  conditions.  This  element  is  not, 
however,  much  used  as  a  colouring  agent  in  practice, 
although  it  has  been  advocated  as  a  "  decolouriser."  The 
writer  is  not,  however,  aware  that  it  has  ever  been  success- 
fully used  for  this  purpose,  and,  in  fact,  the  colours  to  which 
it  gives  rise  do  not  appear  to  be  even  approximately  com- 
plementary to  the  ordinary  green  and  blue  tints  which 
"  decolourisers  "  are  intended  to  cover. 

Cobalt  is  one  of  the  most  powerful  colouring  agents  in 
glass,  and  is  very  largely  used  in  the  production  of  all 
varieties  of  blue  glass.  The  blue  colour  produced  by 
cobalt  is,  in  fact,  probably  the  most  "certain"  of  the 
colours  available  to  the  glass-maker,  this  tint  being  least 
affected  by  all  those  circumstances  that  lead  to  variations 
in  other  tints.  Almost  the  only  difficulty  involved  in  the  use 
of  cobalt  is  the  great  colouring  power  of  this  element,  which 
requires  that  for  most  purposes  only  very  small  quantities 
may  be  added  to  the  glass  mixture.  Formerly  cobalt  was 
added  to  glass  mixtures  in  the  form  of  "  zaffre,"  which  was 
a  very  impure  form  of  cobalt  oxide.  At  the  present  time, 
however,  the  more  expensive  but  much  more  satisfactory 
pure  oxide  of  cobalt  is  in  almost  universal  use.  This 


198  GLASS   MANUFACTURE. 

substance  shows  a  perfectly  constant  composition  and,  by 
means  of  accurate  weighing,  enables  the  glass-maker  to 
introduce  precisely  the  right  amount  of  cobalt  into  his 
batch. 

The  range  of  colours  which  are  available  to  the  modern 
glass  manufacturer  are,  as  will  be  seen  from  a  considera- 
tion of  the  list  of  colouring  elements  given  above,  practi- 
cally unlimited,  particularly  as  these  substances  can  be 
used  in  almost  any  combination  to  produce  mixed  or  inter- 
mediate tints.  This  practically  infinite  variety  of  possible 
tints,  indeed,  involves  the  principal  difficulty  encountered  by 
the  manufacturer  of  coloured  glass,  i.e.,  that  of  matching 
his  tints,  or  of  keeping  the  colour  of  any  particular  variety 
of  glass  so  constant  that  pieces  produced  at  various  times 
can  be  used  indiscriminately  together.  This  ideal  is, 
perhaps,  never  entirely  realised,  but  in  the  case  of  glasses 
intended  for  special  technical  uses  the  ideal  degree  of 
constancy  is  very  closely  approached. 

In  addition  to  being  called  upon  to  produce  a  large 
variety  of  different  tints,  the  glass-maker  is  also  called 
upon  to  produce  various  depths  of  the  same  tint.  In  many 
cases  this  can  be  readily  done  by  the  simple  means  of 
varying  the  amount  of  colouring  material  added  to  the 
glass.  Where  the  colouring  effect  of  small  quantities  of 
these  substances  is  not  excessively  powerful  there  is  no 
very  great  difficulty  in  doing  this,  but  in  certain  cases  this 
mode  of  regulating  the  intensity  of  the  colour  is  not 
available.  Thus  copper-ruby  glass  cannot  readily  be  made 
of  so  light  a  tint  as  to  appear  of  reasonable  depth  when  used 
in  sheets  of  the  thickness  of  ordinary  sheet-glass.  As  has 
already  been  indicated,  the  desired  tint  is  obtained  by  the 


COLOUBED  GLASSES.  199 

process  of  "  flashing,"  i.e.,  of  placing  a  verythin  layer  of  deep 
ruby-coloured  glass  upon  the  surface  of  a  sheet  of  ordinary 
more  or  less  colourless  glass  of  the  usual  thickness.  This  is 
generally  accomplished  by  having  a  pot  of  molten  ruby 
glass  available  close  to  a  pot  from  which  colourless  glass  is 
being  gathered.  A  small  gathering  of  ruby  glass  is  first 
taken  up  on  the  pipe,  and  the  remaining  gatherings  required 
for  the  production  of  the  sheet  are  taken  from  the  pot  of 
colourless  glass.  When  such  a  composite  gathering  is 
blown  into  a  cylinder  in  the  manner  described  in  the 
previous  chapter,  the  ruby  glass  lies  as  a  thin  layer  over 
the  inner  face  of  the  cylinder,  but  special  care  and  skill  on 
the  part  of  the  gatherer  and  blower  is  required  to  ensure 
that  this  layer  shall  be  evenly  distributed  and  of  the 
right  thickness  to  produce  just  the  tint  of  ruby  required. 
Since  the  whole  layer  of  red  glass  is  so  thin,  a  very  slight 
want  of  uniformity  in  its  distribution  leads  to  wide  varia- 
tions of  tint,  and  in  practice  these  are  often  seen  in  the 
less  successful  cylinders  of  such  glass. 

The  chemical  composition  of  the  ruby  and  the  colourless 
glass  which  are  to  be  employed  for  this  purpose  must  also 
be  properly  adapted  to  one  another  in  order  to  produce  two 
glasses  which  shall  have  as  nearly  the  same  coefficient  of 
thermal  expansion  as  possible.  If  this  requirement  is  not 
met,  the  resulting  glass  is  subjected  to  internal  strains 
which  may  lead  to  fracture,  while,  if  the  ruby  glass  has  the 
higher  co-efficient  of  expansion,  the  sheet  after  flattening 
tends  to  draw  itself  up  on  the  "  flashed  "  side  and  cannot 
be  passed  out  of  the  annealing  kiln  in  a  properly  flat  con- 
dition. 

Although  most  usually  applied  to  copper-ruby  glass,  the 


200  GLASS  MANUFACTURE. 

flashing  process  is  often  used  with  other  colours  also. 
Coloured  glass  of  this  kind  is  at  once  recognised  when 
looked  at  through  the  edges.  Thus  examined  the  glass 
simply  shows  the  greenish  tint  of  ordinary  sheet-glass 
which  constitutes  practically  the  entire  thickness  of  the 
sheet.  In  the  same  way,  if  such  "  flashed  "  glass  be  cut  or 
etched  in  such  a  way  that  the  layer  of  coloured  glass  is 
removed  in  places,  the  resulting  pattern  appears  in  white 
on  the  coloured  ground — a  feature  which  is  utilised  for 
certain  decorative  purposes.  The  flashing  process  just 
described,  it  should  be  noted,  is  applicable  to  any  form  of 
glassware  which  is  blown  from  a  gathering,  and  the 
coloured  layer  can  be  applied  either  upon  the  inside  or 
outside  of  any  object  thus  produced. 

In  addition  to  the  palette  of  colours  which  the  glass- 
maker  is  able  to  supply,  the  artist  in  stained  glass  has  a 
fnrther  range  of  colours  at  his  disposal  in  the  form  of 
stains  and  transparent  colours  which  can  be  applied  to  the 
surface  of  glass  and  developed  and  rendered  more  or  less 
permanent  by  being  properly  "  fired."  The  colours  pro- 
duced in  this  way  are  also,  in  one  sense,  coloured  glasses, 
or  rather  glazes,  whose  raw  materials  are  put  upon  the 
glass  by  the  brush  of  the  painter,  and  only  subsequently 
caused  to  combine  and  melt  by  suitable  heating.  The 
degree  of  heat  applicable  under  these  circumstances  is, 
however,  very  limited  by  the  necessity  of  avoiding  any 
great  softening  of  the  substratum  of  glass,  while  many  of 
the  colours  themselves  are  composed  of  materials  which 
could  not  resist  very  high  temperatures.  The  fluxes  used 
in  the  composition  of  these  colours  must  for  this  reason  be 
of  a  very  fusible  kind,  with  the  inevitable  result  of  a  greatly 


COLOURED   GLASSES.  201 

reduced    chemical    stability  as    compared    with   the  glass 
itself. 

The  whole  subject  of  painting  on  glass,  even  from  the 
purely  technical  as  apart  from  the  aesthetic  point  of  view, 
is  a  very  wide  one,  and  lies  outside  the  scope  of  the  present 
volume.  Only  one  further  technical  point  in  connection 
with  glass-painting  and  stained  glass  work  will  therefore 
be  touched  upon  here.  This  is  an  example  of  the  fact  that 
the  more  technically  "perfect"  modern  product  is  not 
always  preferable  for  special  purposes  which  have  been 
well  served  by  older  and  far  less  "  perfect  "  products.  The 
production  of  technically  excellent  coloured  glass  in  modern 
times  was,  somewhat  surprisingly  at  first,  accompanied  by 
a  very  marked  decline  in  the  artistic  beauty  of  stained  glass 
windows  produced  with  this  modern  material ;  the  ancient 
art  of  stained  glass  was,  therefore,  for  a  time  regarded  as  a 
"  lost  art,"  and  glass-makers  were  blamed  for  being  unable 
to  produce  the  brilliant  and  beautiful  tints  which  had  been 
formerly  available.  More  careful  study,  however,  revealed 
the  fact  that  while  the  actual  colour  of  modern  glass  was  at 
least  as  brilliant  and  varied  as  that  of  ancient  glass,  the 
difference  lay  in  the  fact  that  the  modern  glass  was  practi- 
cally entirely  free  from  such  imperfections  as  air-bubbles, 
striae,  and  other  defects  which  improved  appliances  and 
methods  had  enabled  the  glass-maker  to  eliminate  from  his 
products.  Finding  the  beauty  of  his  wares  greatly  improved 
by  this  increased  purity  of  the  glass  in  the  case  of  window 
glass  and  table  ware,  it  was  natural  for  the  glass-maker  to 
endeavour  to  produce  the  same  "improvement"  in  the 
coloured  glasses  intended  for  artistic  purposes  and,  indeed, 
it  is  more  than  likely  that  the  stained-glass  workers  them- 


202  GLASS  MANUFACTUEE. 

selves  pressed  this  line  of  improvement  upon  him  by  a 
demand  for  "better"  glass.  It  turned  out,  however,  on 
close  examination,  that  this  very  perfection  of  modern 
glass  rendered  it  less  adapted  for  these  artistic  purposes. 
A  perfect  piece  of  glass,  having  smooth  surfaces  and  no 
internal  regularities,  allows  the  rays  of  light  falling  upon 
it  to  pass  through  undeflected  in  direction,  and  merely 
changed  in  colour,  according  to  the  tint  of  the  glass  in 
question.  On  looking  at  the  glass,  external  objects  can  be 
quite  clearly  seen,  and  much  of  the  interest  and  mystery  of 
the  glass  itself  is  lost.  On  the  other  hand,  when  falling 
upon  a  piece  of  glass  having  an  irregular  surface,  and  con- 
taining all  manner  of  irregularities  such  as  striae,  air  bells, 
and  even  pieces  of  enclosed  solid  matter,  the  light  is 
scattered,  refracted,  and  deflected  into  all  manner  of 
directions  until  it  almost  appears  to  emanate  from  the 
body  of  the  glass  itself,  which  thus  appears  almost  to  shine 
with  an  internal  light  of  its  own  ;  the  eye  can  hardly 
perceive  the  presence  of  external  objects,  and  the  whole 
window  appears  as  a  brilliant  self-luminous  object. 

Once  their  attention  had  been  drawn  to  these  facts, 
modern  glass-makers  endeavoured,  and  with  much  success, 
to  reproduce  the  desirable  qualities  of  the  ancient  glass, 
while  still  availing  themselves  of  modern  methods  to  pro- 
duce more  stable  glasses  and  a  wider  range  of  colours. 
The  irregular  surface  of  the  old  glass  is  imitated  by  using 
rolled  or  "  muffed  "  instead  of  ordinary  blown  glass,  while 
the  internal  texture  is  rendered  non-homogeneous  by  the 
deliberate  introduction  of  solid  and  gaseous  impurities  and 
by  manipulations  so  arranged  as  to  leave  the  glass  in  layers 
of  different  density,  which  appear  in  the  finished  glass  as 


COLOUEED  GLASSES.  203 

"  striae."  As  a  consequence,  it  is  probably  not  too  much  to 
claim  that  the  modern  workers  in  coloured  glass  have 
materials  at  their  disposal  which  are  at  least  as  suitable 
for  the  purpose  as  those  that  were  available  in  the  best  days 
of  the  ancient  art. 

Some  reference  has  already  been  made  to  the  technical 
uses  of  coloured  glass,  but  one  or  two  further  points  in  that 
connection  remain  to  be  discussed.  For  such  technical 
purposes  as  railway  and  marine  signals,  the  consensus  of 
practical  experience  has  decided  in  favour  of  certain  colours 
of  glass,  such  as  red  and  green  of  particular  tints.  On  the 
other  hand,  for  various  purposes  in  connection  with  photo- 
graphy, the  glass-maker  does  not  appear  to  have  been  able 
to  meet  the  new  requirements,  with  the  result  that  flimsy 
and  otherwise  unsatisfactory  screens  made  of  gelatine  or 
celluloid  stained  with  organic  dyes  are  employed  in  place 
of  coloured  glass  in  such  cases,  for  example,  as  the  covering 
of  lamps  for  use  in  photographers'  "dark"  rooms,  and  for 
the  light-filters  used  for  orthochromatic  and  tri-chromatic 
photography.  In  all  these  cases  it  is  necessary  to  use  a 
transparent  coloured  medium  which  transmits  only  light  of 
a  certain  very  definite  range  of  wave-lengths,  and  there  is 
no  doubt  that  for  the  glass-maker,  who  is  confined  to  the 
use  of  a  number  of  elementary  bodies  for  his  colouring 
media,  it  is  b}^  no  means  easy  to  comply  with  these  require- 
ments of  exact  transmission  and  absorption.  On  the  other 
hand,  the  field  of  available  coloured  glasses  has  not  been 
fully  explored  from  this  point  of  view,  the  only  extensive 
work  on  the  subject  having  been  done  in  connection  with 
the  Jena  firm  of  Schott,  who  have  put  upon  the  market  a 
series  of  coloured  glasses  of  accurately-known  absorbing 


204     .  GLASS  MANUFACTUEE. 

power.  There  is,  however,  little  doubt  that  a  much  greater 
extension  of  this  field  is  possible,  and  that  it  will  be  opened 
up  by  a  glass-maker  who  undertakes  the  exhaustive  study 
of  coloured  glasses  from  this  point  of  view,  although  it 
must  be  admitted  that  there  is  considerable  doubt  whether 
the  results  obtainable  by  the  aid  of  aniline  and  other  dyes 
as  applied  to  gelatine  can  ever  be  equalled  by  coloured 
glasses. 


CHAPTEK   XII. 

OPTICAL    GLASS. 

OPTICAL  glass  differs  so  widely  from  all  other  varieties  of 
glass  that  its  manufacture  may  almost  be  regarded  as  a 
separate  industry,  to  which,  indeed,  a  separate  volume 
could  well  be  devoted.  In  the  present  chapter  we  propose 
to  give  an  outline  of  the  most  important  properties  of 
optical  glass,  and  in  the  next  chapter  to  describe  the  more 
important  features  of  the  processes  used  in  its  production. 

The  properties  which  affect  the  value  of  optical  glass  may 
roughly  be  divided  into  two  groups.  The  first  group  com- 
prises the  specifically  "optical"  properties — i.e.,  those 
directly  influencing  the  behaviour  of  light  in  its  passage 
through  the  glass,  while  the  second  group  covers  those 
properties  of  a  more  general  nature,  which  are  of  special 
importance  in  glass  that  is  to  be  used  for  optical  purposes. 

Optical  Properties  of  Glass. — The  most  essential  property 
of  glass  in  this  respect  is  homogeneity.  We  have  already 
indicated  that  glass  can  never  be  regarded  as  a  definite 
chemical  substance  or  compound,  but  that  it  usually  con- 
sists of  mutual  solutions  of  various  complex  silicates, 
borates,  etc.  Solutions  being  of  the  very  nature  of 
mixtures  of  two  or  more  different  substances,  it  follows 
that  they  can  only  become  homogeneous  when  complete 


206  GLASS   MANUFACTURE. 

mixing  has  taken  place.  We  have  a  familiar  example  of 
the  formation  of  such  a  solution  when  sugar  is  dissolved  in 
water.  The  water  near  the  sugar  becomes  saturated  with 
sugar  and  of  different  density  from  the  remaining  water ; 
if  the  liquid  is  slightly  stirred  a  very  characteristic 
phenomenon  makes  its  appearance — the  pure  water  and 
the  dense  sugar  solution  do  not  at  once  mix  completely, 
the  denser  liquid  remaining  for  a  time  disseminated  through- 
out the  whole  fluid  mass  in  the  form  of  more  or  less  fine 
lines,  sheets,  or  eddies,  and  these  are  visible  because  the 
imperfectly  mixed  liquids  have  different  effects  on  the  light 
passing  through  them.  In  the  case  of  sugar-water  we  are, 
however,  dealing  with  a  very  mobile  liquid,  and  a  few  turns 
of  a  tea-spoon  suffice  to  render  the  mixture  complete,  and 
the  liquid,  which  for  a  few  moments  had  appeared  turbid, 
becomes  homogeneous  and  transparent.  In  the  case  of 
glass,  when  the  raw  materials  are  melted  together,  a 
mixture  is  formed  of  liquids  of  differing  densities  similar 
to  that  which  was  temporarily  formed  in  the  sugar-water 
solution.  Molten  glass,  however,  is  never  so  mobile  a  liquid 
as  ordinary  water,  nor  is  it  in  the  ordinary  course  of 
manufacture  subjected  to  any  such  thorough  mixing  action 
as  that  which  is  produced  by  a  spoon  in  a  glass  of  water. 
In  glass  as  ordinarily  manufactured,  therefore,  it  is  not 
surprising  to  find  that  the  lack  of  homogeneity  which 
originates  during  the  melting  persists  to  the  end.  Its  effects 
can  be  traced  whenever  a  thick  piece  of  ordinary  glass  is 
carefully  examined,  when  the  threads  or  layers  of  differing 
densities  can  be  recognised  in  the  form  of  minute  internal 
irregularities  in  the  glass.  These  defects  are  known  as 
striae  or  veins,  and  their  presence  in  glass  intended  for  the 


UNIVERSITY 

OF 


OPTICAL   GLASS. 


207 


better  kind  of  optical  work  renders  the  glass  useless.  As 
will  be  seen  below  in  the  production  of  optical  glass,  special 
means  are  adopted  for  the  purpose  of  rendering  it  as 
homogeneous  as  possible ;  in  fact,  the  early  history  of 
optical  glass  manufacture  is  simply  the  history  of  attempts 
to  overcome  this  very  defect.  The  problem  is,  however, 
beset  by  chemical  and  physical  difficulties  of  no  mean 
order,  and  even  in  the  best  modern  practice  only  a  small 
proportion  of  each  melting  or  crucible  full  of  glass  is 


FIG.  14. — Diagram  of  striae-testing  apparatus. 

L,  source  of  light ;  S,  slit;  A  and  Z?,  simple  convex  lenses;  G,  glass  under 
test ;  E,  eye  of  observer.     The  arrows  indicate  the  paths  of  light-rays. 

entirely  free  from  veins  or  striae.  In  many  cases  these 
defects  are  very  minute,  and  sometimes  escape  observation 
until  the  stage  of  the  finished  lens  is  reached.  At  that  stage, 
however,  their  presence  becomes  painfully  evident  from  the 
fact  that  they  interfere  seriously  with  the  sharp  definition 
of  the  images  formed  by  the  lens  in  question.  It  will  be 
seen  that  in  such  a  case  time  and  money  has  been  wasted 
by  grinding  and  polishing  what  turns  out  to  be  a  useless 
piece  of  glass.  Methods  are,  therefore,  used  for  examining 
the  glass  before  it  is  worked,  whereby  the  existence  of  the 
smallest  striae  can  scarcely  escape  detection.  These  methods 


l>08  GLASS  MANUFACTUEE. 

depend  upon  the  principle  that  a  beam  of  parallel  light 
passing  through  a  plate  of  glass  will  meet  with  no  dis- 
turbance so  long  as  the  glass  is  homogeneous,  but  if  striae 
are  present,  they  will  cause  the  light  to  deviate  from 
parallelism  wherever  it  falls  upon  them.  Under  such 
illumination,  therefore,  the  stride  will  appear  as  either  dark 
or  bright  lines,  when  they  can  be  readily  detected.  One 
form  of  apparatus  used  for  this  purpose  is  illustrated  in 
Fig.  14. 

Transparency  and  colour  are  obviously  fundamentally 
important  properties  of  glass.  In  one  sense  homogeneity 
is  essential  to  transparency,  but  the  aspect  of  the  subject 
which  we  are  now  considering  is  that  of  the  absorption  of 
light  in  the  course  of  regular  transmission  through  glass. 
It  may  be  said  at  once  that  no  glass  is  either  perfectly 
transparent  or,  what  comes  to  nearly  the  same  thing,  per- 
fectly free  from  colour.  In  the  case  of  the  best  optical 
glasses  it  is  true  that  the  absorption  of  light  is  very  slight, 
but  even  these,  when  considerable  thicknesses  are  viewed, 
show  a  greenish-yellow  or  bluish  colouring.  On  the  other 
hand,  certain  optical  glasses  which  are  used  at  the  present 
time  for  many  of  our  best  lenses  absorb  light  so  strongly  or 
are  so  deeply  coloured  that  a  thickness  of  a  few  inches  is  suffi- 
cient to  reveal  this  defect.  To  some  extent  public  taste  or 
opinion  which  objects  to  the  use  of  even  a  slightly  greenish 
glass  in  optical  instruments  of  good  quality  is  to  blame  for 
the  tint  of  these  glasses.  In  man}'  cases  glass-makers 
could  produce  a  very  slightly  greenish  glass,  but  in  order  to 
overcome  this  colour  they  deliberately  add  to  the  glass  a 
colouring  oxide  imparting  to  the  glass  a  colour  more  or  less 
complementary  to  the  natural  green  tint.  The  result  is  a 


OPTICAL  GLASS.  209 

more  or  less  neutral-tinted  glass  which,  however,  absorbs 
much  more  light  than  the  naturally  green  glass  would  have 
done.  Since  such  glass  is  frequently  used  for  photographic 
lenses,  it  is  interesting  to  note  that  the  light  rays  whose 
transmission  is  sacrificed  in  order  to  avoid  the  green  tint 
are  those  lying  at  or  near  the  blue  end  of  the  spectrum,  so 
that  the  photographic  rapidity  of  the  resulting  lenses  is 
decidedly  reduced  by  the  use  of  such  glass. 

Refraction  and  Dispersion. — The  quantitative  properties 
of  glass,  governing  its  effect  upon  incident  and  transmitted 
light,  are,  of  course,  of  fundamental  importance  in  all  its 
optical  uses.  The  fundamental  optical  constant  of  each 
variety  of  optical  glass  is  known  as  its  refractive  index ; 
this  number  really  represents  the  ratio  of  the  velocity  with 
which  light  waves  are  propagated  through  the  glass  to  the 
velocity  with  which  they  travel  through  free  space.  Not 
only  does  this  ratio  vary  with  every  change  in  the  chemical 
composition  and  physical  condition  of  the  glass,  but  it  also 
varies  according  to  the  length  of  the  light  waves  them- 
selves. In  other  words,  the  short  waves  of  blue  light  are 
transmitted  through  glass  with  a  different  velocity  from 
that  with  which  the  longer  waves  of  red  light  are  trans- 
mitted. The  consequence  is  that  when  a  beam  of  white 
light  is  passed  through  a  prism  it  is  split  up  and  spread 
out  into  a  number  of  beams  representing  all  the  colours  of 
the  spectrum  in  their  proper  order,  the  blue  light  suffering 
the  greatest  deflection  from  its  original  path,  while  the  red 
light  suffers  least  deflection.  Both  the  actual  and  relative 
amount  by  which  light  rays  of  various  colours  are  deflected 
under  such  circumstances  depends  upon  the  nature  of  the 
glass  in  question ;  therefore,  to  fully  characterise  the 

G.M.  p 


210  GLASS   MANUFACTURE. 

optical  properties  of  a  given  kind  of  glass  it  is  necessary  to 
state  not  only  its  refractive  index  but  to  specify  the  refrac- 
tive indices  for  a  sufficient  number  of  different  wave-lengths 
of  light,  suitably  distributed  through  the  spectrum.  For 
this  purpose  a  number  of  well-marked  spectrum  lines  have 
been  chosen,  the  systematic  use  of  the  particular  set  of 
lines  which  is  now  usually  employed  being  due  to  the 
initiative  of  Abbe  and  Schott  at  Jena,  who  initiated  the 
system  of  specifying  the  optical  properties  of  glass  in  this 
way.  The  actual  lines  chosen  are  the  line  known  as  A' , 
corresponding  to  a  wave-length  of  0'7677  micro-millimetres, 
and  the  lines  known  as  C,  D,  F,  and  G',  whose  wave- 
lengths, in  the  same  units,  are  0*6563,  0*5893,  0'486'2,  and 
0'4341  respectively.  The  A'  line,  however,  lies  so  near  the 
extreme  red  end  of  the  spectrum  that  the  data  concerning 
it  are  seldom  required. 

As  a  matter  of  fact,  the  actual  refractive  index  is  only 
stated  in  most  tables  of  optical  glasses  for  sodium  light 
(D  line),  the  dispersive  properties  of  the  glass  being  indicated 
by  tabulating  the  differences  between  the  refractive  indices 
for  the  various  lines,  the  table  thus  containing  columns 
marked  C-D,  D-F,  F-G'.  These  figures  are  usually 
described  as  the  **  dispersion  "  of  the  glass  from  C  to  D,  D 
to  F,  etc.  In  addition  to  these  figures  it  is  usual  to  tabulate 
what  is  called  the  "  mean  dispersion  "  of  the  glass,  which 
is  simply  the  difference  between  the  refractive  indices  for 
C  and  F  lines  ;  this  interval  is  usually  taken  as  repre- 
senting that  part  of  the  spectrum  which  is  of  the  greatest 
importance  for  visual  purposes.  A  further  constant  which 
is  of  great  importance  in  the  calculations  for  achromatic 
lenses  is  obtained  by  dividing  the  mean  dispersion  into  the 


OPTICAL   GLASS. 


211 


refractive  index  for  the  D  line  minus  one  (usually  written 

f-\ -p 

-=v).      This  term,  for  which  no  satisfactory   name 


has  yet  been  suggested,  characterises  the  ratio  of  the  dis- 
persive power  of  the  glass  to  its  total  refracting  power.  It 
is  usually  denoted  hy  the  Greek  letter  v.  The  following 
table  (taken  from  the  Catalogue  of  the  Optical  Convention, 
1905)  gives  a  list  of  optical  glasses  produced  by  Messrs. 
Chance,  of  Birmingham.  This  list,  although  it  is  not 
nearly  so  long  as  that  issued  by  the  French  and  German 
firms  who  manufacture  optical  glass,  contains  examples  of 
the  most  important  types  of  optical  glass  which  are  avail- 
able at  the  present  time.  Those,  however,  who  wish  to  use 
the  data  for  the  purpose  of  lens  calculation  are  advised  to 
consult  the  latest  issues  of  the  optical  glass-makers'  cata- 
logues, since  the  range  of  types  available,  and  even  the 
actual  figures  for  some  of  the  glasses,  are  liable  to  variation 
from  time  to  time. 

In  the  table  on  p.  212  the  first  column  contains  the 
ordinary  trade  names  by  which  the  various  types  of  glass  are 
known.  These  names,  while  somewhat  arbitrary,  indicate 
in  a  rough  way  the  chemical  nature  of  the  glass  concerned. 
Thus  the  word  "  flint  "  always  implies  a  glass  containing 
lead  and  therefore  having  a  comparatively  high  refractive 
index  and  low  value  of  v,  while  the  word  "  crown,"  originally 
applied  only  to  lime-silicate  glasses,  is  now  used  for  all 
glass  having  a  high  value  of  v.  In  the  next  column  of  the 
table  are  given  the  refractive  indices  of  the  glasses,  while 
the  third  column  contains  the  values  of  v.  It  will  be  seen 
that  the  glasses  are  arranged  in  descending  order  of  magni- 


212 


GLASS  MANUFACTUEE. 


O  O  Tt<  t-  CO  O 


^D  CO  rH 
•^  OS  <7<1 

1C  »O  O  CO  CO 


8888888888 


O  O  O  T— I  CO 

T— IT— IT— I  r— I  T— I 


•^OCOTjHOODTFT— (i— lt»THrHCC 
OOOOOOOOi— lOT-iTHT-i 


TJH  CO  O  OS  CO  rfl  O 

pppppppppppppppppppppp 


•^t-OOCO(NCO<MCOX(M(M^OT-ICOT— I  O 
-  O5OST-l(MTticO^TtHt-OOt-O5OS^t- 


O  fc-<N  OS  t> 


OPTICAL   GLASS.  213 

tude  in  respect  of  this  constant.  An  inspection  of  the 
figures  in  these  two  columns  will  reveal  the  fact  that  for 
the  majority  of  the  glasses  contained  in  this  table  the 
value  of  v  decreases  as  the  refractive  index  increases. 
The  glasses  which  are  an  exception  to  this  rule  are  indicated 
by  an  *.  Asa  matter  of  fact  this  rule  applied  to  all  glasses 
that  were  known  or  were  at  all  events  commercially  avail- 
able prior  to  the  modern  advances  in  optical  glass  manufac- 
ture which  were  initiated  by  Abbe  and  Schott  of  Jena.  It 
was  Abbe's  insight  into  the  requirements  of  optical  instru- 
ment design  that  led  him  to  realise  the  importance  of 
overcoming  this  limitation  in  the  ratio  between  the  disper- 
sive and  refractive  powers  of  glass.  With  the  collaboration 
of  Schott  he  succeeded  in  producing  a  whole  series  of 
previously  unknown  varieties  of  optical  glass  in  which  the 
relation  between  n  and  v  is  not  that  of  approximately 

simple  inverse  proportionality  which  holds  for  the  older 
crown  and  flint-glasses.  Most  valuable  and  in  many  ways 
most  typical  of  these  new  glasses  are  those  known  as  the 
"  barium  crown  "  glasses,  which  combine  the  high  refractive 
index  of  a  light  flint  or  even  a  dense  flint-glass  with  the 
high  v  value  of  an  ordinary  crown  glass.  It  would  lead 
too  far  into  the  subject  of  lens  construction  to  explain  in 
detail  the  possibility  opened  up  to  the  optician  by  the  use 
of  these  newer  varieties  of  glass.  We  must  content  our- 
selves with  pointing  out  that  the  great  forward  strides 
marked  by  the  production  of  apochromatic  microscope 
objectives,  of  anastigmatic  photographic  lenses,  and  the 
modern  telescope  objectives  are  all  based  upon  the  employ- 
ment of  these  new  optical  media;  and  although  optical 


214  GLASS  MANUFACTURE. 

glasses  of  these  newer  types  are  at  the  present  time  pro- 
duced in  the  optical  glass  manufactories  of  France  and 
England,  in  quality  and  quantity  at  least  equal  to  the  out- 
put of  the  Jena  works  themselves,  these  great  optical 
achievements  stand  as  a  lasting  monument  to  the  pioneer 
work  of  Abbe  and  Schott  in  this  field. 

The  last  six  columns  of  the  table  of  optical  glasses  given 
above  contain  figures  which  define  the  manner  in  which 
each  of  the  glasses  named  distributes  the  various  sections 
of  the  spectrum.  The  columns  C-D,  D-F,  and  F  to  G' 
give  as  already  indicated  the  differences  between  the 
refractive  indices  for  the  C,  D,  F  and  G'  lines  respectively  ; 
the  smaller  figures  in  the  intermediate  columns  indicate  the 
ratio  of  each  of  these  differences  to  the  mean  dispersion  of 
the  glass.  If  all  kinds  of  glass  distributed  the  various 
portions  of  the  spectrum  in  the  same  proportionate  manner, 
merely  differing  in  the  total  amount  of  dispersion  produced, 
these  figures  would  be  identically  the  same  for  all  glasses. 
In  actual  fact  it  will  be  seen  that  the  figures  differ  very 
widely  from  one  type  of  glass  to  another.  A  moment's 
consideration  will  show  that  when  two  glasses  are  used  in  a 
lens  for  the  purpose  of  achromatising  one  another,  i.e.,  when 
one  is  used  to  neutralise  the  dispersion  of  the  other,  such 
achromatisation  can  only  be  perfect  if  these  ratios  (the 
relative  partial  dispersions)  are  the  same  for  both  glasses. 
To  put  the  same  statement  in  more  concrete  terms,  if  the 
spectrum  produced  by  one  glass  is  comparatively  long 
drawn  out  at  the  red  end,  relatively  compressed  at  the  blue 
end,  while  in  the  other  glass  the  opposite  relation  holds 
between  the  two  ends  of  the  dispersion  spectrum,  it  is 
evident  that  the  two  spectra  can  never  be  superposed  in 


OPTICAL  GLASS. 


215 


such  a  way  as  to  entirely  neutralise  one  another — the 
spectrum  produced  by  the  one  glass  will  predominate  and 
leave  a  residual  colour  at  the  blue  end,  while  the  other  will 
predominate  at  the  other  end.  In  the  case  of  lenses 
achromatised  by  the  use  of  such  glasses,  there  will  always 
be  a  slight  fringe  of  colour  around  the  borders  of  the 
images  which  they  produce.  One  of  the  aims  which  Abbe 
and  Schott  set  themselves  in  the  production  of  new  varieties 
of  optical  glass  was  to  obtain  one  or  more  pairs  of  glasses 
in  which  the  relative  partial  dispersions  should  be  as  nearly 
alike  as  possible  while  the  actual  values  of  v  should  differ  as 
widely  as  possible.  Some  success  in  this  direction  was  at 
first  claimed  by  the  Jena  workers,  but  unfortunately  some 
of  the  most  promising  glasses  in  this  respect  were  found  to 
be  too  unstable  for  practical  use  and  had  ultimately  to  be 
abandoned.  At  the  present  time  the  only  pair  of  really 
perfectly  achromatic  glasses  offered  by  the  Jena  firm  is 
that  tabulated  below,  and  it  will  be  seen  that  although  the 
relative  partial  dispersions  are  very  closely  alike,  the  v 
values  of  the  two  glasses  only  differ  by  10,  and  at  least  one 


c-d 

d-f 

f 

Name. 

llD 

v 

C-F. 

C-D. 

<TT. 

D-F. 

F-G'. 

tt 

Telescope  Crown 

1-5254 

61-7 

•00852 

•00250 

••202 

•00602 

•707 

•00484 

•568 

Telescope  Flint  . 

1-5-211 

51-8 

•001007 

•00297 

•294 

•00710 

•705 

•00577 

•573 

of  these  glasses  is  not  readily  obtainable  in  really  satisfac- 
tory optical  quality.  On  the  other  hand,  practically  per- 
fectly achromatised  lenses  (generally  known  as  "  apochro- 
matic  ").  have  been  produced,  especially  by  Zeiss  of  Jena, 
for  microscopic  purposes,  by  the  careful  selection  of  glasses 


216  GLASS  MANUFACTUEE. 

suited  to  each  other  in  this  respect.  Such  a  solution  of 
the  problem  is  further  facilitated  by  the  fact  that  in  these 
lenses  more  than  two  varieties  of  glass  can  be  used  to 
neutralise  one  another,  while  a  natural  mineral  (fluorite)  is 
also  employed.  From  the  glass-maker's  point  of  view, 
however,  the  problem  of  producing  a  satisfactory  pair  of 
glasses  capable  of  entirely  achromatising  one  another  has 
yet  to  be  solved. 

The  table  of  optical  glasses  given  above,  although  brief 
as  compared  with  the  lists  issued  by  French  and  German 
optical  glass-makers,  fairly  covers  the  range  of  practically 
available  glasses,  and  a  rapid  inspection  will  at  once  show 
how  extremely  limited  this  range  really  is.  Thus  the 
refractive  index  varies  only  between  the  limits  1'49  and 
1*71,  and  even  if  we  admit;  as  practical  glasses  such  extreme 
types — offered  by  some  makers — as  would  extend  this 
range  to  1*40  in  one  direction  and  to  1*80  in  the  other, 
this  does  not  affect  the  present  argument.  Of  course,  a 
glass  of  a  refractive  index  as  low  as  I/O,  or  even  I'lO,  is 
not  theoretically  possible,  since  the  mere  density  of  any 
substance  enters  into  the  factors  that  affect  its  refractive 
index,  and  a  glass  having  a  density  lower  than  that  of 
water  (whose  refractive  index  is  about  1/8)  is  scarcely  con- 
ceivable. In  the  other  direction,  however,  the  limits  met 
with  in  the  case  of  glass  are  considerably  exceeded  by 
certain  natural  mineral  substances.  Thus  the  diamond  has 
a  refractive  index  of  2*42,  while  the  garnets  show  refractive 
indices  from  1/75  to  1*81.  The  values  of  v  found  in  the 
table  of  optical  glasses  are  still  more  narrowly  restricted, 
lying  between  67  and  29,  while  such  a  mineral  as  fluorite 
ghows  a  value  of  95'4.  These  facts  show  that  it  is  physi- 


OPTICAL   GLASS.  217 

cally  possible  to  obtain  transparent  substances  having 
optical  properties  lying  far  beyond  the  limited  range 
covered  by  our  present  optical  glasses,  and  it  scarcely 
needs  showing  that  if  such  an  extended  range  of  materials 
were  available  greatly  increased  possibilities  would  be 
opened  up  to  the  designer  of  optical  instruments.  It  is 
consequently  interesting  to  inquire  as  to  the  actual  causes 
which  limit  the  range  of  optical  glasses  at  present  available. 
It  will  be  found  that  these  limits  are  set  by  the  properties 
of  glass  itself.  '  While  the  more  ordinary  kinds  of  glass, 
having  average  optical  properties  and  showing  dispersive 
powers  roughly  conforming  to  the  law  of  inverse  propor- 
tionality with  refractive  index  which  governs  the  older 
varieties  of  optical  glass,  are  chemically  stable  substances, 
showing  little  tendency  to  undergo  either  chemical  changes 
or  to  crystallise  during  cooling,  the  more  extreme  glasses 
exhibit  these  undesirable  features  to  an  increasing  extent 
the  more  nearly  the  limit  of  our  present  range  is  approached. 
As  the  chemical  composition  of  a  glass  is  "  forced  "  by  the 
addition  of  special  substances  intended  to  affect  its  optical 
properties  in  an  abnormal  direction,  so  the  chemical  and 
physical  stability  of  the  glass  is  rapidly  lessened.  The  more 
extreme  glasses,  in  fact,  behave  as  active  chemical  agents 
readily  entering  into  reaction  or  combination  even  with 
relatively  inert  substances  in  their  environment — they  act 
vigorously  upon  the  fire-clay  vessels  in  which  they  are 
melted,  and  they  are  readily  attacked  by  acids,  moisture  or 
even  warm  air,  when  in  the  finished  condition,  while  many 
of  them  can  only  be  prevented  from  assuming  the  condition 
of  a  crystalline  (and  opaque)  agglomerate  by  being  rapidly 
Cooled  through  certain  critical  ranges  of  temperature. 


218  GLASS  MANUFACTURE. 

A  limit  to  the  possibility  of  production  is  set  by  these 
tendencies  when  they  exceed  a  certain  amount — a  point 
being  reached  where  it  ceases  to  be  practicable  to  overcome 
the  tendency  of  the  glass  to  self-destruction.  On  the  lines 
of  our  present  glasses,  therefore,  it  does  not  appear  hopeful 
to  look  for  any  considerable  extension  of  the  range  of  our 
optical  media.  On  the  other  hand,  as  the  known  optical 
properties  of  transparent  crystalline  minerals  show,  a  much 
greater  range  of  optical  constants  would  become  available 
if  it  were  possible  to  manufacture  artificial  mineral  crystals 
of  sufficient  size  and  purity  for  optical  purposes,  and  the 
author  believes  that  in  this  direction  progress  in  optical 
materials  is  ultimately  bound  to  lie1. 

In  addition  to  possessing  the  requisite  optical  constants, 
a  good  colour  and  perfect  homogeneity,  certain  other 
properties  are  essential  in  good  optical  glass.  These  are 
the  general  physical  and  chemical  qualities  which  are 
essential  in  all  good  glass,  but  especially  emphasised  by 
the  fact  that  the  requirements  for  optical  glass  are  more 
stringent  than  for  any  other  variety  of  the  material.  Thus 
chemical  stability  is  of  the  greatest  importance,  for  the  best 
lenses  would  soon  become  useless  if  the  action  of  atmo- 
spheric moisture  were  to  affect  them  appreciably — the 
polished  surfaces  would  rapidly  become  dull  and  the  whole 
lens  would  soon  be  rendered  useless.  The  conditions 
governing  the  chemical  stability  of  glass  and  the  methods 
of  testing  this  quality  have  already  been  indicated  (Chapters 
I.  and  II.) .  The  harder  varieties  of  optical  glass,  such  as 

1  See  a  Paper  by  the  present  author  on  "  Possible  Directions  of 
Progress  in  Optical  Glass  " — Proceedings  of  the  Optical  Convention, 
London,  1905. 


OPTICAL   GLASS. 


219 


the  glasses  quoted  in  the  above  table  under  the  names  of 
"  Hard  Crown "  and  Boro-Silicate  Crown,  are  probably 
among  the  most  durable  and  chemically  resistant  of  all 
varieties  of  glass,  but  as  we  have  already  indicated,  when 
extreme  optical  properties  are  required,  the  necessary 
chemical  composition  of  the  glass  always  entails  a  sacrifice 
of  this  great  chemical  stability,  until  a  limit  is  reached 
where  valuable  optical  properties  no  longer  counterbalance 
the  serious  disadvantage  of  a  chemical  composition  which 
renders  the  glass  liable  to  rapid  disintegration.  In  certain 
special  cases  it  is,  perhaps,  possible  to  protect  lenses  made 
of  such  unstable  glass  by  covering  them  with  cemented-on 
lenses  of  stable  glass,  but  this  device  entails  concomitant 
limitations  in  the  design  of  the  optical  system  and  is,  there- 
fore, rarely  used.  In  any  case,  however,  it  is  well  for  the 
lens-designer  to  consider  the  relative  stability  of  the  glasses 
employed  when  arranging  the  order  in  which  they  are  to 
be  used,  since  it  is  obviously  preferable  to  put  a  hard, 
durable  glass  on  the  outside  of  his  system,  where  it  is  most 
directly  exposed  to  atmospheric  moisture,  and  is  also  subject 
to  handling  and  "  cleaning  "  by  inexpert  hands.  This  latter 
factor  is  a  very  important  one  for  the  life  of  any  lens.  In 
the  first  place,  a  glass  surface  is  very  seriously  affected  by 
the  minute  film  of  organic  matter  which  is  left  upon  it 
when  it  has  been  touched  with  even  a  clean  finger ;  unless 
the  glass  is  of  the  best  quality  in  this  respect,  such  finger- 
marks readily  develop  into  iridescents  pots  and  may  even 
turn  into  black  stains.  Particles  of  dust  allowed  to  settle 
on  the  surface  of  the  glass  will  affect  it  in  the  same  way,  so 
that  the  protection  afforded  by  mere  mechanical  enclosure 
in  the  tube  of  an  instrument  is  of  decided  value  in 


220  GLASS  MANUFACTURE. 

preserving  a  glass  surface.  It  should,  however,  be  noted 
that  in  some  instances  the  interior  metal  surfaces  of  optical 
instruments  are  varnished  with  substances  that  give  off 
vapours  for  a  long  time  after  the  instrument  is  completed, 
and  in  that  case  the  inside  lenses  are  apt  to  be  tarnished  in 
consequence.  On  the  other  hand,  outside  lenses  are  also 
exposed  to  direct  mechanical  injury  from  handling  and 
"cleaning."  As  far  as  the  latter  operation  is  concerned,  it 
frequently  happens,  particularly  in  glasses  containing  soda, 
that  a  slight  surface  dimming  is  formed  on  the  glass  when 
it  has  been  left  in  a  more  or  less  damp  place  for  a  long 
time.  This  dimming  is  chiefly  due  to  the  formation  on  the 
surface  of  a  great  number  of  very  minute  crystals  of 
carbonate  of  soda,  which  are  hard  and  sharp  enough  to 
scratch  the  glass  itself  if  rubbed  about  over  it.  If  such  a 
lens  be  wiped  with  a  dry  cloth,  however  clean  and  soft,  the 
effect  is  a  permanent  injury  to  the  polished  surface,  which 
could  readily  be  avoided  by  first  washing  the  lens  with  clean 
water,  or  even  by  using  a  wet  cloth  instead  of  a  dry  one 
for  the  first  wiping. 

The  mechanical  hardness  of  the  glass  is  an  important 
factor  in  determining  its  resistance  to  such  injurious  treat- 
ment or  to  the  effects  of  accidental  contact  with  hard,  sharp 
bodies.  The  subject  of  the  hardness  of  glass  has  already 
been  discussed  in  a  general  way  in  Chapter  II.,  and  little 
remains  to  be  added  here.  Broadly  speaking,  a  high  degree 
of  hardness  and  a  low  refractive  index  are  found  together. 
This  statement  is  certainly  true  where  any  considerable 
difference  of  hardness  is  considered,  as,  for  example,  in 
comparing  a  hard  crown  glass  with  a  dense  flint ;  but  where 
the  difference  of  refractive  index  or  of  density  is  small,  it 


OPTICAL  GLASS.  221 

is  not  at  all  certain  that  the  lighter  glass  will  also  be  the 
harder. 

The  properties  involved  in  the  quality  known  as  "  hard- 
ness "  also  affect  in  a  very  marked  manner  the  behaviour 
of  glass  when  subjected  to  the  grinding  and  polishing 
processes.  The  ease  with  which  a  good  polish  can  be 
obtained  varies  very  much  in  different  kinds  of  glass,  both 
the  hardest  and  the  softest  glasses  showing  themselves 
difficult  in  this  respect.  The  harder  glasses  are  certainly 
less  liable  to  accidental  scratching  during  the  polishing 
operations,  and  generally  work  in  a  cleaner  manner;  but 
the  time  required  to  produce  a  satisfactory  polish  is  much 
greater  owing  to  the  resistance  to  displacement  offered  by 
the  molecules.  Both  the  speed  of  working  and  the  pressure 
exerted  during  the  polishing  operation  have,  in  fact,  to  be 
carefully  adapted  to  the  quality  of  the  glass  in  this  respect 
if  the  best  possible  results  are  to  be  obtained. 

Another  property  which  is  essential  in  optical  glass  of 
the  highest  quality  is  that  of  freedom  from  internal  strains. 
This  subject  will  be  again  referred  to  later  in  connection 
with  the  annealing  processes  used  in  the  manufacture  of 
optical  glass,  and  it  need  only  be  mentioned  here  that  the 
presence  of  internal  strain  is  readily  recognised  in  glass, 
by  the  aid  of  the  polariscope.  Perfectly  annealed 
glass,  entirely  free  from  internal  strains,  produces  no  effect 
upon  a  beam  of  polarised  light  passing  through  it,  while 
even  slightly  strained  glass  becomes  markedly  doubly- 
refracting.  For  many  purposes  of  optics  this  double 
refraction  becomes  undesirable  or  even  inadmissible, 
especially  as  it  is  accompanied  by  small  variations  in  the 
effective  index  of  refraction  of  various  portions  of  the  mass 


222  GLASS  MANUFACTUEE. 

of  glass.  Further,  if  the  amount  of  double  refraction 
observed  is  at  all  serious  it  indicates  a  state  of  strain  which 
may  easily  lead  to  the  fracture  of  the  whole  piece,  particu- 
larly when  undergoing  the  earlier  stages  of  the  grinding 
process  or  if  exposed  to  shocks  of  any  sort.  As  will  be 
seen  below,  perfectly  annealed  glass  is  obtainable,  but  very 
special  means  are  required  for  its  production,  and  the 
optician  should  for  that  reason  avoid  making  unnecessarily 
extreme  demands  in  this  direction.  The  very  small  amount 
of  double  refraction  frequently  found  in  the  better  class  of 
optical  glass  is  entirely  harmless  for  most  purposes. 


CHAPTER    XIII. 

OPTICAL    GLASS. 

THE  process  of  manufacturing  the  best  qualities  of 
optical  glass  may  be  briefly  described  as  consisting  in 
obtaining  a  crucible  full  of  the  purest  and  most  homo- 
geneous glass,  and  then  allowing  it  to  cool  slowly  and  to 
solidify  in  situ.  From  the  resulting  mass  of  glass  the  best 
pieces  are  picked  and  moulded  into  the  desired  shape  for 
optical  use.  It  will  be  seen  at  once  that  in  this  process 
there  is  an  essential  difference  from  all  others  that  have 
been  described  in  this  book — viz.,  that  the  glass  is  never 
removed  from  the  melting-pot  while  molten,  and  that  none 
of  the  operations  of  gathering,  pouring,  rolling,  pressing,  or 
blowing  are  applied  to  it.  The  reason  for  this  apparently 
irrational  mode  of  procedure  lies  in  the  fact  that  the  perfect 
homogeneity  essential  for  optical  purposes  can  only  be 
attained  by  laborious  means,  and  can  then  only  be  retained 
if  the  glass  is  left  to  solidify  undisturbed ;  any  movement 
by  the  introduction  of  pipes  or  ladles  would  result  in  the 
contamination  of  the  glass  by  striae  and  other  objectionable 
defects. 

The  choice  and  proportion  of  raw  materials  used  in  the 
production  of  any  given  quality  of  optical  glass  is  governed 
by  the  chemical  composition  which  experiment  has  shown 


224  GLASS  MANUFACTUKE. 

to  be  necessary  to  yield  the  desired  optical  properties.  The 
composition  of  optical  glass  mixtures  cannot  therefore  be 
varied  to  suit  the  conditions  of  the  furnace  or  to  facilitate 
ready  melting  and  fining,  so  that  many  of  the  usual 
resources  of  the  glass-maker  cease  to  be  available  in  the 
very  case  where  their  aid  would  be  most  welcome  to 
facilitate  the  production  of  technically  perfect  glass.  On  the 
other  hand,  the  manufacturer  has  a  certain  amount  of 
choice  as  to  the  precise  form  in  which  the  various  chemical 
ingredients  are  to  be  introduced  into  the  mixture,  and  he 
makes  his  choice  among  oxides,  carbonates,  nitrates,  and 
hydrates,  according  to  the  behaviour  that  it  is  desired  to 
impart  to  the  mass  during  the  earlier  stages  of  fusion. 
The  state  of  purity  in  which  the  various  substances  are 
commercially  obtainable  also  enters  largely  into  the 
question,  since  the  greatest  possible  degree  of  purity  in 
the  raw  materials  is  essential  to  the  production  of  glass  of 
good  colour,  or  rather  freedom  from  colour. 

Since  homogeneity  is  so  essential  in  the  finished  product, 
very  thorough  mixing  of  the  raw  materials  is  necessary  in 
the  case  of  optical  glass,  and  the  ingredients  are  for  this 
purpose  generally  used  in  a  state  of  finer  division  than  is 
necessary  with  other  varieties  of  glass.  As  a  rule  the 
quantities  of  mixture  of  any  one  kind  that  are  required  are 
not  large  enough  to  justify  the  use  of  mechanical  appliances, 
and  very  careful  hand-mixing  is  carried  out. 

Although  it  is  quite  possible  to  obtain  successful  meltings 
from  raw  materials  alone,  it  is  preferable  to  mix  with  these 
a  certain  proportion  of  "  cullet  "  or  broken  glass  derived 
from  a  previous  melting  of  the  same  sort.  The  broken 
glass  used  for  this  purpose  is  first  carefully  picked  over  for 


OPTICAL   GLASS.  225 

the  purpose  of  rejecting  pieces  that  contain  visible  impuri- 
ties, although  pieces  showing  striae  are  not  usually  rejected. 
The  greater  part  of  this  cullet  is  generally  mixed  as 
evenly  as  possible  with  the  raw  materials,  but  a  certain 
proportion  is  reserved  for  another  purpose,  as  explained 
below. 

The  furnaces  used  for  the  production  of  optical  glass  vary 
very  much  in  type  in  different  works.  In  some  the  old- 
fashioned  conical  coal  furnaces  are  still  used,  the  disadvan- 
tages attached  to  their  employment  being  outweighed — in 
the  opinion  of  the  manufacturers — by  their  simplicity  and 
ease  of  regulation.  In  other  works  gas -fired  regenerative 
furnaces  of  the  most  recent  type  are  installed,  and  in  these 
also  optical  glass  of  the  highest  quality  can  be  produced. 
As  a  rule,  however,  optical  glass  furnaces  differ  from  other 
pot-furnaces  found  in  glass-works  in  this  respect — that  the 
former  are  usually  constructed  to  receive  one  pot  or  crucible 
only,  while  in  other  glass  furnaces  from  four  to  twelve  or 
even  twenty  pots  are  heated  at  the  same  time.  The  reason 
for  this  restriction  in  the  capacity  of  the  furnaces  lies  in 
the  fact  that  since  the  mixtures  used  for  optical  glass  can- 
not be  adjusted  to  suit  the  furnace,  the  latter  must  be 
worked  as  far  as  possible  in  such  a  way  as  to  suit  the 
mixture  to  be  melted  in  it,  and  this  implies  that  every  pot 
will  require  its  own  adjustment  of  times  and  temperatures, 
and  this  it  would  be  difficult,  if  not  impossible,  to  secure  if 
more  than  one  pot  were  heated  in  the  same  furnace.  It  is 
further  to  be  remembered  that  the  amount  of  care  and 
attention  required  during  the  melting  of  a  pot  of  optical 
glass  is  out  of  all  proportion  to  that  needed  with  other 
varieties,  so  that  little  would  be  gained  by  having  a  number 

G.M.  Q 


226  GLASS   MANUFACTUEE. 

of  pots  in  one  furnace,  since  several  sets  of  men  would 
be  required  to  tend  them. 

In  addition  to  the  single-pot  melting  furnace,  a  very 
important  part  of  the  equipment  of  the  optical  glass  works 
is  formed  by  a  number  of  kilns  or  ovens  which  are  used  for 
the  preliminary  heating,  and  sometimes  for  the  final  cool- 
ing of  the  various  crucibles  or  pots.  Similar  kilns  are 
used  in  other  branches  of  the  industry,  but  in  those  cases 
the  pots,  once  introduced  into  the  furnace,  are  expected  to 
last  for  a  number  of  weeks,  or  even  months.  In  optical 
glass  manufacture,  on  the  other  hand,  a  pot  is  used  once 
only,  so  that  fresh  pots  are  required  for  every  new  melting. 
The  kilns  in  which  these  pots  are  heated  up  before  being 
placed  in  the  melting  furnace  are  thus  in  very  frequent  use. 
As  a  rule  they  are  simply  fire-brick  chambers  provided 
with  sufficient  grate-room  and  flue-space  to  be  gradually 
raised  to  a  red  heat  in  the  course  of  four  or  five  days, 
while  for  the  purpose  of  gradual  cooling  they  can  be  sealed 
up  like  the  annealing  kilns  used  for  polished  plate-glass. 

The  pots  or  crucibles  in  which  optical  glass  is  melted  are 
usually  of  the  same  shape  as  the  covered  pots  used  for  flint- 
glass  as  illustrated  in  Fig.  2.  The  optical  glass  pots, 
however,  are  made  considerably  thinner  in  the  wall,  since 
they  are  not  required  to  withstand  the  prolonged  action  of 
molten  glass  in  the  same  way  as  pots  used  for  flint-glass 
manufacture.  On  the  other  hand,  the  fire-clays  used  for 
this  purpose  must  be  chosen  with  special  care  so  as  to  avoid 
any  contamination  of  the  glass  by  iron  or  other  impurities 
which  might  reach  the  glass  from  the  pot.  For  the  pro- 
duction of  certain  special  glasses,  in  fact,  pots  made  of 
special  materials  are  required,  since  these  glasses,  when 


OPTICAL   GLASS.  227 

molten,  produce  a  rapid  chemical  attack  upon  ordinary  fire- 
clays. A  certain  amount  of  the  aluminiferous  material  of 
the  pot  is,  in  fact,  always  introduced  into  the  glass  by  the 
gradual  dissolving  action  of  glass  on  fire-clay  which  we 
have  already  described.  The  glass  contaminated  with 
these  aluminiferous  substances  is  generally  more  viscous 
than  the  rest  of  the  contents  of  the  pot,  and  therefore 
ordinarily  remains  more  or  less  adherent  to  the  walls  of  the 
crucible,  but  the  inevitable  disturbances  which  accompany 
the  processes  of  melting  and  fining  lead  to  the  dissemina- 
tion of  some  of  this  viscous  glass  through  the  entire  pot  in 
the  form  of  veins  or  striae,  which  are  only  removed  during 
the  stirring  process.  On  the  other  hand,  more  of  this 
viscous  glass  is  constantly  being  formed  so  long  as  the 
glass  remains  molten,  and  if  disturbances  are  not  sufficiently 
avoided  during  the  later  stages  of  the  process  fresh  veins 
may  easily  be  formed. 

The  actual  operations  of  producing  a  melting  of  optical 
glass  begin  by  the  gradual  heating-up  of  the  pot  in  the 
kiln  just  described.  When  the  pot  has  reached  a  full  red. 
heat  the  doors  of  the  kiln  are  opened  and  the  pot  drawn 
out  by  means  of  a  long  heavy  iron  fork  running  on  wheels ; 
this  implement  is  run  into  the  mouth  of  the  kiln  and  the 
tines  of  the  fork  are  pushed  under  the  pot,  and  the  latter  is 
then  readily  lifted  up  and  withdrawn  from  the  kiln.  Mean- 
while the  temperature  of  the  furnace  has  been  regulated  in 
such  a  manner  as  to  be  approximately  equal  to  that 
attained  by  the  heating  kiln,  so  that  the  pot,  when  trans- 
ferred as  rapidly  as  possible  from  the  kiln  to  the  furnace, 
is  not  subjected  to  any  very  sudden  heating ;  were  it 
attempted  to  place  the  new  pot  in  a  furnace  at  full  melt- 
ed 2 


228  GLASS  MANUFACTUKE. 

ing  heat  the  fire-clay  would  shrink  rapidly  and  the  entire 
vessel  would  fall  to  pieces.  Even  under  the  best  conditions 
it  is  not  possible  to  avoid  the  occasional  failure  of  a  pot  by 
cracking  either  at  this  or  a  slightly  later  stage  of  the 
process.  The  latter  occurrence  is  apt  to  be  particularly 
disastrous,  as  the  pot  may  then  be  full  of  molten  glass, 
which  runs  out  and  is  lost. 

As  soon  as  the  empty  pot  has  been  put  into  place,  the 
melting  furnace  is  carefully  sealed  up  by  means  of  tempo- 
rary work  built  of  large  fire-bricks,  the  whole  being  so 
arranged  that  the  mouth  of  the  hood  of  the  pot  is  left 
accessible  by  means  of  an  aperture  in  the  temporary 
furnace  wall.  This  aperture  can  be  closed  by  one  or  more 
slabs  of  fire-clay,  and  when  these  are  removed  an  opening  is 
left  by  which  the  raw  materials  are  introduced,  and  through 
which  the  other  manipulations  are  carried  out. 

When  this  stage  of  the  process  is  reached,  the  wagons 
containing  the  mixed  raw  materials  are  usually  wheeled 
into  place  in  front  of  the  furnace,  but  the  introduction  of 
the  materials  themselves  into  the  pot  is  not  begun  until 
several  hours  later,  when  the  furnace  has  been  vigorously 
heated  and  an  approach  to  the  melting  heat  has  been 
attained. 

When  the  furnace  and  pot  have  attained  the  necessary 
temperature,  but  before  the  raw  materials  are  introduced, 
a  small  quantity  of  the  cullet,  which  has  been  reserved  for 
this  purpose,  is  thrown  into  the  pot  and  allowed  time  to 
melt,  and  then  only  is  the  first  charge  of  mixture  put  into 
the  pot.  The  object  of  this  proceeding  is  to  coat  the  bottom 
and  part  of  the  walls  of  the  pot  with  a  layer  of  molten  glass 
which  serves  to  protect  it  from  the  chemical  and  physical 


OPTICAL  GLASS.  229 

attack  of  the  raw  materials  during  the  violent  action  which 
takes  place  when  they  are  first  exposed  to  the  furnace  heat. 

The  gradual  filling  of  the  pot  with  molten  glass  is  now 
carried  out  by  the  introduction  of  successive  charges  of  raw 
material ;  as  the  mixture  not  only  occupies  more  space  than 
the  glass  it  forms,  but  also  froths  up  a  good  deal  during 
melting,  the  quantities  introduced  each  time  must  be  care- 
fully adjusted  so  as  to  avoid  an  overflow  of  half-melted  glass 
through  the  mouth  of  the  pot.  As  the  pot  is  more  and 
more  nearly  filled,  the  space  left  for  the  raw  materials  is 
proportionately  diminished,  and  the  later  charges  are  there- 
fore much  smaller  than  the  first  few. 

When,  finally,  sufficient  material  has  been  introduced  to 
fill  the  pot  completely,  the  next  stage  of  the  process 
commences.  When  the  last  charge  of  raw  materials  has 
melted,  the  glass  in  the  pot  is  left  in  the  state  of  a  more  or 
less  viscous  liquid  full  of  bubbles  of  all  sizes ;  it  is  essential 
that  these  bubbles  should  escape  and  leave  the  glass  pure 
and  "fine,"  and  this  result  can  only  be  achieved  by  raising 
the  temperature  of  the  furnace  and  allowing  the  glass  to 
become  more  fluid,  while  the  rise  of  temperature  also  causes 
the  bubbles  to  expand  owing  to  the  expansion  of  the  gas 
contained  in  them.  In  both  ways,  rise  of  temperature 
facilitates  the  escape  of  the  bubbles,  and  the  furnace  is 
therefore  heated  to  the  full,  and  this  extreme  heat  is 
maintained  until  the  glass  is  free  from  bubbles.  Jn  the 
case  of  the  more  fusible  glasses  the  temperature  required 
for  this  purpose  is  not  excessively  high,  and,  indeed,  in  the 
case  of  these  glasses  care  is  taken  to  avoid  too  high  a 
temperature,  as  it  entails  other  disadvantages.  In  the  case 
of  the  harder  crown  glasses,  however,  the  difficulty  lies  in 


230  GLASS  MANUFACTUBE. 

producing  an  adequately  high  temperature  without  at  the 
same  time  endangering  the  life  of  furnace  and  crucible. 
The  difficulty  of  freeing  the  molten  glass  from  bubbles 
constitutes  one  of  the  causes  that  limit  the  range  of  our 
optical  glasses  in  one  direction — still  harder  glasses  could 
be  melted,  but  it  would  not  be  feasible  to  maintain  a 
temperature  high  enough  to  render  them  fluid  enough 
to  "fine." 

In  the  case  of  other  kinds  of  glass,  again,  it  becomes 
impossible  to  entirely  remove  the  bubbles  from  the  molten 
mass  even  when  very  hot  and  very  fluid.  The  exact  cause 
is  not  known,  but  in  some  kinds  of  glass  the  bubbles 
formed  are  so  minute  that  even  when  the  glass  is  perfectly 
mobile  the  bubbles  show  no  tendency  to  escape,  while  in 
other  kinds  of  glass  there  appears  to  be  a  steady  evolution 
of  minute  bubbles  as  soon  as  the  temperature  is  raised  with 
a  view  to  removing  those  already  in  the  glass.  As  this 
property  attaches  to  some  of  the  most  valuable  of  the  newer 
varieties  of  optical  glass,  opticians  and  the  public  have 
learnt  to  put  up  with  the  presence  of  minute  bubbles  in  the 
lenses  and  prisms  made  of  these  glasses.  These  bubbles 
are,  however,  very  minute  and  do  not  interfere  with  the 
optical  performance  of  the  lenses,  &c.,  except  to  the  extent 
of  arresting  and  scattering  the  very  small  proportion  of 
light  that  falls  upon  them  ;  their  presence  is  therefore  to 
be  regarded  as  a  small  but  unavoidable  drawback  to  the 
use  of  glasses  which  offer  advantages  that  completely  out- 
weigh this  defect. 

Returning  to  the  melting  process,  we  find  that  the 
extreme  heating  required  for  the  purpose  of  "  fining"  the 
glass  is  continued  for  a  considerable  period  of  time,  as  long 


OPTICAL  GLASS.  231 

as  thirty  hours  in  some  cases,  the  glass  being  examined  from 
time  to  time  to  test  its  condition  as  regards  freedom  from 
bubbles.  This  is  done  by  taking  a  small  sample  of  glass 
out  of  the  pot  and  examining  it  to  see  if  it  still  contains 
bubbles.  In  some  works  this  test  is  made  by  taking  up  a 
very  small  gathering  of  glass  on  the  end  of  a  small  pipe  and 
blowing  it  into  a  spherical  flask  ;  on  looking  at  such  a  flask 
in  a  suitable  light  the  presence  of  even  minute  bubbles  is 
readily  detected.  In  other  works  a  simpler  process  is 
adopted,  a  small  quantity  of  glass  being  ladled  out  of  the 
pot  on  the  surface  of  a  flat  iron  rod.  It  is  allowed  to  cool 
on  the  rod,  and  when  pushed  off  forms  a  small  bar  of  glass 
some  eight  or  ten  inches  long  and  about  an  inch  wide ;  in 
this  also  the  presence  of  bubbles  is  easily  detected.  These 
test  pieces  are  known  among  glass-makers  as  "  proofs." 

When  proofs,  taken  as  just  described,  have  shown  that 
the  glass  is  free  from  bubbles,  the  extreme  heat  of  the 
furnace  is  allowed  to  abate,  and  the  fire-clay  slabs  in  front 
of  the  mouth  of  the  pot  are  removed.  The  next  step  is 
that  of  skimming  the  surface  of  the  glass.  Since  most  of 
the  materials  liable  to  contaminate  the  contents  of  a  pot 
are  specifically  lighter  than  the  molten  glass,  they  will  be 
found  floating  on  the  surface,  and  the  surface  glass  is  there- 
fore removed  with  a  view  to  ridding  the  glass  of  anything 
that  may  have  been  accidentally  introduced  and  that  has 
not  melted  and  become  incorporated  with  the  molten  mass. 

The  next  steps  in  the  process  are  those  of  stirring  the 
molten  glass  with  a  view  to  rendering  it  homogeneous  and 
free  from  striae.  The  stirrer  used  for  this  purpose  is 
usually  a  cylinder  of  fire-clay,  previously  burnt  and  heated. 
This  is  provided  with  a  deep  square  hole  in  one  end,  and  it 


232  GLASS  MANUFACTURE. 

is  held  at  first  by  means  of  a  small  iron  bar  passed  into 
this  hole.  By  this  means  the  red-hot  cylinder  of  fire-clay 
is  introduced  into  the  open  mouth  of  the  pot,  and  when  it 
has  attained  approximately  the  temperature  of  the  molten 
glass  it  is  dipped  into  the  glass  itself,  in  which  it  ultimately 
floats.  When  stirring  is  to  begin,  the  square,  down-turned 
end  of  a  long  iron  bar  is  introduced  into  the  corresponding 
square  hole  in  the  upper  end  of  the  stirrer,  and  by  this 
means  the  fire-clay  cylinder  is  held  in  a  vertical  position  in 
the  glass  and  given  the  steady  rotatory  movement  which 
constitutes  the  stirring  process.  For  this  purpose  the  long 
iron  bar  just  mentioned  is  made  to  pass  over  a  swivel- 
wheel,  while  a  workman  moves  it  steadily  by  the  aid  of  a 
large  wooden  handle.  This  operation  is  always  laborious 
and  trying  ;  the  workman  is  necessarily  exposed  to  the 
intense  heat  radiated  from  the  open  mouth  of  the  crucible, 
so  that  men  have  to  relieve  each  other  at  frequent  intervals. 

During  the  earlier  stages  of  the  stirring  process  the  glass 
is  very  hot  and  mobile,  but  the  stirring  is  continued,  with 
short  intervals,  until  the  glass  is  so  cold  and  stiff  that  the 
stirrer  can  scarcely  be  moved  in  it  at  all,  so  that  the  work 
of  moving  the  stirrer  becomes  heavy  towards  the  end  of 
the  operation.  The  actual  amount  of  stirring  required 
varies  according  to  the  nature  of  the  glass,  and  the  size  of 
the  pot  or  crucible  in  question.  Some  meltings  are  found 
to  be  satisfactory  after  as  little  as  four  hours'  stirring,  while 
for  others  as  much  as  20  hours  are  required. 

When  the  glass  has  stiffened  to  such  an  extent  that  it  is 
no  longer  possible  to  continue  the  stirring,  preparations  are 
made  for  the  final  cooling-down  of  the  pot  of  glass.  The 
fire-clay  stirrer  is  sometimes  withdrawn  from  the  glass,  but 


OPTICAL  GLASS.  233 

this  is  laborious,  and  entails  dragging  a  considerable 
quantity  of  glass  out  of  the  pot  with  the  clay  cylinder  ; 
more  usually,  therefore,  the  stirrer  is  simply  left  embedded 
in  the  glass. 

The  next  object  to  be  accomplished  is  that  of  cooling  the 
glass  as  rapidly  as  safety  will  permit  until  it  has  become 
definitely  "  set " — the  purpose  being  to  prevent  the  recru- 
descence of  striae  as  a  result  of  convection  currents  or  other 
causes  which  might  disturb  the  homogeneity  of  the  glass. 
This  rapid  cooling  is  obtained  in  various  ways;  in  one 
mode  of  procedure  the  furnace  is  so  arranged  that  by 
opening  a  number  of  apertures  provided  for  the  purpose 
cold  air  is  drawn  in  and  the  pot  and  its  contents  chilled 
thereby  without  being  moved.  This  method  has  the 
advantage  that  the  pot  containing  the  viscous  glass  is 
never  moved  or  disturbed  in  any  way,  but  on  the  other 
hand  the  cooling  which  can  be  effected  within  the  furnace 
itself  is  never  very  rapid,  and  the  furnace  as  well  as  the  pot 
is  chilled.  Further  when  the  glass  has  been  chilled  down 
to  a  certain  point  this  rapid  rate  of  cooling  must  be 
arrested,  as  otherwise  the  whole  contents  of  the  pot  would 
crack  and  splinter  into  minute  fragments.  Where  the  pot 
has  been  left  in  the  furnace  this  can  only  be  done  by 
sealing  up  the  whole  furnace  with  temporary  brickwork 
and  lutings  of  fire-clay,  leaving  it  to  act  as  an  annealing 
kiln  until  the  glass  has  cooled  down  approximately  to  the 
ordinary  temperature,  a  process  that  occupies  a  period 
of  from  one  to  two  weeks  according  to  the  size  of  the 
melting.  Such  enforced  idleness  of  a  melting  furnace  is  of 
course  very  undesirable  from  an  economical  point  of  view, 
and  it  is  generally  avoided  by  adopting  the  alternative 


234  GLASS  MANUFACTUKE. 

method  of  drawing  the  pot  bodily  out  of  the  furnace  as 
soon  as  the  stirring  operation  is  ended.  For  this  purpose 
the  temporary  brickwork  forming  the  front  of  the  furnace 
is  broken  down,  and  with  the  aid  of  a  long  crow-bar  the 
bottom  of  the  pot  is  levered  up  from  the  bed  or  siege  of  the 
furnace  to  which  it  adheres  strongly,  being  bound  down  by 
the  sticky  viscous  mass  of  molten  glass  and  half-molten 
fire-clay  which  always  accumulates  on  the  bed  of  the 
furnace.  The  pot  being  temporarily  held  up  by  the  inser- 
tion of  a  piece  of  fire-brick,  the  tines  of  a  long  and  heavy 
iron  fork  running  on  a  massive  iron  truck  are  introduced 
beneath  the  pot ;  an  iron  band  provided  with  long  handles 
is  then  passed  around  the  pot,  and  the  latter  is  then  drawn 
forward  by  the  aid  of  suitable  pulley  blocks.  The  tines  of 
the  fork  are  then  raised,  and  the  pot  is  wheeled  out  of  the 
furnace  and  deposited  upon  a  suitable  support.  Here  it  is 
allowed  to  cool  to  the  requisite  extent,  when  it  is  again  picked 
up  on  the  tines  of  the  fork  and  deposited  in  an  annealing 
kiln  which  has  been  previously  warmed  to  a  suitable  tempera- 
ture. It  will  be  seen  that  this  handling  of  a  heavy  mass  of 
intensely  hot  material  involves  much  labour,  while  there  is 
also  a  risk  of  losing  the  glass  if  the  pot  should  break  before 
the  glass  has  set  sufficiently.  Every  care  is  taken  to 
prevent  such  an  accident,  the  pot  being  wrapped  round 
with  chains  or  otherwise  supported  in  such  a  way  that 
a  small  crack  could  not  readily  develop  into  a  large  gap. 

When  such  a  melting  of  glass  has  cooled  sufficiently, 
either  in  the  furnace  or  in  the  annealing  kiln,  to  be  safely 
handled,  the  whole  pot  is  drawn  out,  and  the  fire-clay  shell, 
which  is  generally  found  cracked  into  many  pieces,  is 
broken  away  by  the  aid  of  a  hammer.  Under  favourable 


OPTICAL  GLASS.  235 

circumstances  the  whole  of  the  glass  may  have  cooled 
intact  as  one  solid  lump  sometimes  weighing  over  half 
a  ton.  Unless  special  care  is  taken,  however,  it  is  more 
usual  to  find  the  glass  more  or  less  fissured,  a  number  of 
large  lumps  being  accompanied  by  a  great  mass  of  small 
fragments.  These  are  now  picked  over,  and  all  those  which 
are  free  from  visible  imperfections  or  which  can  be  readily 
detached  from  such  imperfections  by  the  aid  of  a  chipping 
hammer  are  put  upon  one  side  for  further  treatment. 

The  next  step  of  this  treatment  consists  in  moulding  the 
rough  broken  lump  into  the  shape  of  plates,  blocks,  or 
discs  according  to  the  purpose  for  which  the-  glass  may  be 
required  by  the  optician.  The  plant  used  for  the  moulding 
process  varies  widely,  but  in  all  cases  the  operation  con- 
sists in  gradually  heating  the  glass  in  a  suitable  kiln  until 
it  is  soft  enough  to  adapt  itself  to  the  shape  of  the  mould 
provided  for  the  purpose.  In  some  cases  these  moulds  are 
made  of  fire-clay,  and  the  glass  is  simply  allowed  to  settle 
into  them  by  its  own  weight ;  in  other  cases  iron  moulds 
are  used,  and  the  glass  is  worked  into  them  by  the  aid  of 
gentle  pressure  from  wood  or  metal  moulding  tools.  In 
yet  other  cases,  particularly  where  the  glass  is  required  in 
the  form  of  small  thin  discs  or  where  it  is  to  be  formed 
into  the  approximate  shape  of  concave  or  convex  lenses,  the 
aid  of  a  press  is  sometimes  invoked. 

In  all  cases  the  moulding  process  is  followed  by  the  final 
annealing,  which  consists  in  cooling  the  glass  very  gradually 
from  the  red  heat  at  which  it  has  been  moulded,  down  to 
the  ordinary  temperature.  The  length  of  time  occupied  by 
such  cooling  depends  very  much  upon  the  size  of  the  object 
and  also  upon  the  degree  of  refinement  to  which  it  is 


236  GLASS   MANUFACTURE. 

necessary  to  carry  the  removal  of  small  internal  strains  in 
the  glass.  For  many  purposes  it  is  sufficient  to  allow  it  to 
cool  down  naturally  in  a  large  lain  in  the  course  of  six  or 
eight  days.  For  special  purposes,  however,  where  perfect 
freedom  from  double  refraction  is  demanded,  much  greater 
refinements  are  required,  and  special  annealing  kilns,  whose 
temperature  can  be  accurately  regulated  and  maintained, 
are  employed.  In  these  the  annealing  operation  can  be 
carried  out  so  gradually  that  a  rate  of  cooling  in  which  a 
fall  of  1°  C.  occupies  several  hours  can  be  maintained,  so 
that  very  perfectly  annealed  glass  can  be  produced  even  in 
discs  or  blocks  of  large  size. 

When  removed  from  the  annealing  kiln  the  plates  or 
discs  of  optical  glass  are  taken  to  a  grinding  or  polishing 
workshop,  where  certain  of  their  faces  or  edges  are  ground 
and  polished  in  such  a  way  as  to  permit  of  the  examination 
of  the  glass  for  bubbles,  striae  and  other  defects  in  the 
manner  indicated  in  the  previous  chapter.  As  the  amount 
of  sorting  that  can  be  done  while  the  glass  is  still  in  rough 
fragments  is  necessarily  very  limited,  it  follows  that  a  con- 
siderable proportion  of  the  glass  which  has  been  moulded 
and  annealed  must  be  rejected  as  useless  when  thus  finally 
examined.  A  yield  of  perfect  optical  glass,  amounting  to 
10  or  at  most  20  per  cent,  of  the  total  contents  of  each 
pot,  is  therefore  all  that  can  be  expected,  and  smaller  yields 
are  by  no  means  infrequent — a  consideration  that  will 
serve  to  explain  the  relatively  high  price  of  optical  as 
compared  with  other  varieties  of  glass. 

A  consideration  of  the  various  factors  that  are  involved 
in  the  production  of  a  piece  of  perfect  optical  glass  will 
make  it  apparent  that  the  cost  and  difficulty  of  its  pro- 


OPTICAL   GLASS.  237 

duction  increases  rapidly  with  the  weight  of  the  piece  to  be 
produced,  so  that  it  is  not  surprising  to  find  that  the  price 
of  very  large  discs  of  perfect  optical  glass  such  as  those 
required  for  large  astronomical  telescopes,  reaches  figures 
which  become  prohibitive  when  very  large  sizes  are  con- 
sidered. Thus,  while  it  is  quite  possible  to  obtain  say  100 
pounds  of  good  glass  from  a  single  melting  if  the  glass  is 
to  be  used  in  the  form  of  pieces  not  weighing  more  than 
five  or  six  pounds  each,  it  is  only  rarely  that  a  single  block 
of  perfect  glass  can  be  found  weighing  100  pounds.  In  the 
former  case  the  best  pieces  can  be  picked,  the  worst  defects 
can  be  eliminated  by  chipping  the  rough  fragments,  and  at  a 
later  stage  other  defective  pieces  can  be  cut  off  or  ground 
away ;  not  so  where  a  large  single  block  is  required.  A  single 
fine  vein,  perhaps  too  small  to  be  visible  to  the  unaided  eye, 
may  be  found  to  run  through  a  whole  block  in  such  a  way 
that  it  cannot  be  removed  without  breaking  or  cutting  up 
the  whole  piece,  and  it  will  be  seen  that  the  frequency  with 
which  this  is  liable  to  occur  increases  with  the  volume  of 
the  piece  required.  The  difficulties  of  re-heating  and 
moulding  are  also  increased  enormously  with  the  size  of 
the  individual  pieces  of  glass  that  have  to  be  dealt  with, 
and  where  very  large  pieces  have  to  be  heated  and  cooled 
accidental  breakage  becomes  a  serious  risk.  In  view  of 
these  difficulties  it  is  not  surprising  to  find  that  the 
dimensions  of  our  astronomical  refractors  appear  to  have 
approached  their  limit,  but  rather  are  we  led  to  admiration 
of  the  skill  and  enterprise  that  has  pushed  this  limit  so  far 
as  to  produce  discs  of  optical  glass  measuring  as  much  as 
one  metre  in  diameter. 


CHAPTEE    XIV. 

MISCELLANEOUS    PKODUCTS. 

THE  field  of  glass-manufacture  is  so  wide  and  the 
number  and  variety  of  its  products  so  great,  that  in  the 
limited  compass  of  this  volume  it  is  impossible  to  fully 
enumerate  them  all ;  there  are,  however,  a  certain  number 
of  these  products  which,  while  of  considerable  importance 
in  themselves,  yet  do  not  fall  readily  under  any  of  the 
headings  of  the  preceding  chapters.  A  short  space  will 
therefore  be  devoted  to  some  of  these  in  this  place. 

Glass  Tubing. — A  widely-useful  form  of  glass  is  that  of 
tubes  of  all  sizes  and  shapes,  ranging  from  the  fine 
capillary  tubes  used  in  the  construction  of  thermometers  to 
the  heavy  drawn  or  pressed  pipes  that  have  been  employed 
for  drainage  and  other  purposes.  The  process  of  manu- 
facture employed  varies  according  to  the  size  and  nature  of 
the  tube  that  is  required.  Thus  lamp-chimneys  are  really 
a  variety  of  tube,  used  in  short  lengths  and  made  of 
relatively  wide  diameter  and  thin  walls.  These  are  not, 
however,  ordinarily  made  in  the  form  of  long  tubes  cut 
into  short  sections,  but — as  has  already  been  mentioned— 
they  are  blown  into  moulds  in  the  form  of  a  thin-walled 
cylindrical  bottle,  whose  neck  and  bottom  are  subsequently 
removed.  By  this  process  the  various  forms  of  chimneys 


MISCELLANEOUS  PEODUCTS.  239 

for  oil-lamps,  having  contractions  at  certain  parts  of  their 
length,  can  be  readily  produced. 

The  articles  more  strictly  described  as  glass  tubes  are, 
however,  produced  by  a  process  in  which  actual  blowing 
plays  only  a  very  minor  part.  A  gathering  of  suitable  size 
is  taken  up  on  a  pipe,  a  very  small  interior  hollow  space  is 
produced  by  blowing  into  the  pipe,  and  then  the  gathering 
is  elongated  by  swinging  the  pipe  in  a  suitable  manner. 
The  end  of  the  elongated  gathering  furthest  from  the  pipe 
is  then  attached  to  a  rod  or  "  pontil "  held  by  a  second 
workman,  and  the  two  men  then  proceed  to  move  apart, 
drawing  out  the  gathering  of  glass  between  them.  Accord- 
ing to  the  bore  and  thickness  of  wall  required  in  the  tube, 
the  men  regulate  the  speed  at  which  they  move  apart;  the 
thinner  the  tube  is  to  be  the  more  rapidly  they  move,  in 
order  to  draw  the  glass  out  to  a  sufficient  extent  before  it 
hardens  too  much.  The  rate  of  drawing  must,  of  course, 
also  be  adapted  to  the  nature  of  the  glass  in  question,  and 
this  will  vary  very  widely.  For  the  production  of  the 
smaller  bored  tubes  the  men  find  it  necessary  to  separate 
at  a  smart  trot,  while  heavy  tubes  such  as  are  used  for 
gauge-glasses,  are  drawn  of  hard  glass  by  a  very  gradual 
movement.  In  some  cases,  the  setting  of  the  glass,  when 
the  tube  has  attained  the  desired  thickness,  is  hastened  by 
the  aid  of  an  air-blast,  or — in  more  primitive  fashion — by 
boys  waving  fans  over  the  hot  glass.  In  any  case,  suitable 
troughs  are  provided  for  receiving  the  tube  when  drawn, 
and  from  these  the  tube  is  taken  to  an  annealing  kiln  to 
undergo  this  necessary  operation. 

The  glass  used  for  the  production  of  tubing  varies  very 
widely  according  to  the  purpose  for  which  the  product  is 


240  GLASS  MANUFACTUKE. 

intended.     Almost  any  of  the  more  usual  varieties  of  glass 
can  be  readily  drawn  out  into  tubes,  and  the  choice  of  the 
kind  of   glass  to  be  employed   is  therefore   left  to  other 
considerations.     Tubing  required  for  the  use  of  the  lamp- 
worker,  i.e.,  for  the  production   of   instruments  or  other 
articles  by  the  aid  of  the  glass-blower's  blow-pipe,  must 
have  the  capacity  of  undergoing  repeated  cooling  and  heat- 
ing without  showing  signs  of  crystallisation  (devitrification), 
while  reasonable  softness  in  the  flame  is  also  required.  For 
this  purpose,  also,  glass  containing  lead  is  not  admissible, 
since  this  would  blacken  under  the  influence  of  the  blow- 
pipe flame.     Soda-lime  glasses  rather   rich    in   alkali  are 
most  frequently  used  for  these  purposes ;  one  consequence 
of  their  chemical  composition,  however,  is  that  such  glass 
tends  to  undergo  decomposition  when  stored  for  any  length 
of  time,  more  especially  in  damp  places.     Frequently  this 
decomposition  only  manifests  itself  on  heating  the  glass  in 
a  flame,  when  it  either  flies  to  pieces  or  turns  dull  and 
rough  on  the  surface.     Such  glass  is  sometimes  said  to 
have  "  devitrified,"  but  this  is  not  really  the  case;  what 
has  actually  happened  is  that  the   atmospheric  moisture 
has  penetrated  for  some  little  distance  into  the  thickness  of 
the  glass,  probably  hydrating  some  of  the  silica ;  on  heat- 
ing, this  moisture  is  driven  off,  with  the  result  that  either 
a  few  large  cracks,  or  innumerable  fine  ones,  are  formed. 
In  the  latter  case  these  do  not  readily  disappear  when  the 
glass  is  softened  and  the  dull,  rough  surface  is  left  at  the 
end  of  the  operation. 

For  purposes  where  the  glass  is  to  be  exposed  to  high 
temperatures,  tubing  made  of  so-called  "  hard  glass  "  is 
employed.  This  is  practically  a  form  of  Bohemian  crystal 


MISCELLANEOUS  PBODUCTS.  241 

glass,  the  chemical  composition  being  that  of  a  potash-lime 
glass  rather  rich  in  lime.  To  some  extent  this  Bohemian 
hard  glass  has  been  superseded  by  the  special  "combustion 
tube  "  glass  manufactured  by  Schott,  of  Jena.  This  is  a 
very  refractory  borosilicate  glass  containing  some  mag- 
nesia; it  certainly  withstands  higher  temperatures  than 
hard  Bohemian  glass,  and  is  rather  less  sensitive  to  changes 
of  temperature ;  on  the  other  hand,  it  has  the  inconvenient 
property  of  showing  a  white  opalescence  when  it  has  once 
been  heated,  and  this,  after  a  time,  renders  the  glass 
completely  opaque. 

For  many  purposes,  where  heat-resisting  qualities  are 
chiefly  required,  ordinary  glass  has  now  a  formidable  rival 
in  the  shape  of  vitrified  silica,  which  is  now  available  as  a 
satisfactory  commercial  product.  This  substance  offers  the 
great  advantage  that  for  most  ordinary  purposes  it  may  be 
regarded  as  entirely  infusible,  since  the  intense  heat  of  an 
oxygen-fed  flame  is  required  to  soften  or  melt  the  silica. 
Further,  vitreous  silica  has  an  extremely  low  coefficient  of 
expansion,  and  appears  also  to  have  a  rather  high  coefficient 
of  thermal  conductivity.  The  result  is  that  tubes  and  other 
articles  made  of  this  material  possess  an  astonishing 
amount  of  thermal  endurance  (see  Chapter  II.). 

A  white-hot  tube  or  rod  of  this  material  can  be  plunged 
into  cold  water  with  impunity,  and  no  special  care  need  be 
exercised  in  heating  or  cooling  articles  made  of  this  sub- 
stance, unless  articles  of  great  size  and  thickness  are 
involved,  and  even  with  these  only  little  caution  is  needed. 
The  only  disadvantages  which  must  be  balanced  against  the 
great  advantages  just  named  lie  in  the  relatively  high  cost 
of  the  articles  and  in  their  somewhat  sensitive  behaviour  to 

G.M.  R 


242  GLASS  MANUFACTUBE. 

certain  chemical  influences.  As  regards  cost,  vitreous  silica 
is  at  present  available  in  two  different  forms ;  in  the  first 
form  it  resembles  ordinary  glass  very  closely  in  appearance, 
the  shape  and  finish  of  the  tubes  and  vessels  of  this  kind 
having  undergone  very  great  improvements  quite  recently. 
This  silica  glass  has,  in  fact,  been  worked  from  molten 
silica  in  a  way  more  or  less  analogous  to  that  in  which 
ordinary  glass  is  worked,  the  great  extra  cost  of  the  silica- 
ware  being  due,  in  part,  at  all  events,  to  the  extremely  high 
temperature  required  for  melting  and  working  this  material ; 
ordinarily,  in  the  production  of  the  class  of  silica  ware  now 
referred  to,  this  heat  is  generated  by  the  liberal — and 
therefore  expensive — use  of  oxygen  gas.  In  great  contrast 
to  this  glass-like,  transparent  silica  ware  is  the  other  form 
in  which  this  material  is  available.  This  is  a  series  of 
products  obtained  from  the  fusion  of  silica  in  special  forms 
of  electric  furnace  ;  in  this  ware  the  minute  bubbles  so 
readily  formed  in  the  fusion  of  all  forms  of  quartz  are  not 
even  partially  eliminated,  and  by  their  presence — often  in 
the  form  of  long-drawn-out,  capillary  hollows — they  impart 
to  this  ware  its  very  characteristic  milky  appearance.  The 
price  of  this  product,  which  is  mostly  used  in  the  form  of 
tubes,  although  such  articles  as  basins,  crucibles,  and  even 
muffles  of  considerable  size  are  available,  is  much  lower 
than  that  of  the  transparent  variety,  being  in  fact  decidedly 
lower  than  that  of  the  best  porcelain  ;  on  the  other  hand, 
even  this  price  is  considerably  above  that  of  the  best  glass 
tubing. 

Apart  from  the  question  of  cost,  the  use  of  silica  ware  is 
further  limited  by  its  sensitiveness  to  all  forms  of  basic 
materials.  Thus  alkaline  solutions  cannot  be  allowed  to 


MISCELLANEOUS   PRODUCTS.  243 

come  into  contact  with  this  substance,  since  they  attack  it 
vigorously,  especially  when  warm.  At  high  temperatures 
all  basic  materials  produce  a  rapid  attack  on  silica  ware,  the 
silica,  in  fact,  behaving  as  a  strongly  acid  body  at  and  above  a 
red  heat.  The  attack  which  occurs  when  such  a  substance  as 
iron  or  copper  oxide  is  allowed  to  come  into  contact  with 
heated  vitrified  silica  is,  in  fact,  so  rapid  that  a  tube  is 
completely  destroyed  in  a  few  minutes,  the  formation  of 
silicates  resulting  in  the  cracking  and  disintegration  of  the 
whole  piece.  While,  therefore,  silica  ware,  especially  in  its 
cheaper  forms,  undoubtedly  possesses  great  advantages  and 
possibilities,  its  use  must  be  carried  on  with  careful  reference 
to  its  chemical  nature. 

Vitreous  silica,  in  addition  to  the  uses  and  advantages 
just  named,  has  also  an  interest  from  the  optical  point  of 
view ;  this  arises  from  the  fact  that  it  is  transparent  to 
short  (ultra-violet)  light  waves  to  which  all  ordinary 
varieties  of  glass  are  completely  opaque.  Quite  recently, 
the  Jena  works  have  produced  special  glasses  which  are 
more  transparent  to  these  ultra-violet  rays  than  ordinary 
glass,  but  even  these  fall  far  short  of  silica  in  this  respect. 
This  property  of  transparence  to  ultra-violet  light  is  utilised 
in  two  widely  different  directions.  One  of  these  is  in  the 
production  of  ultra-violet  light  when  required  for  medical  or 
other  special  purposes ;  a  most  energetic  source  of  such 
rays  is  available  by  the  use  of  tubes  of  vitrified  silica  within 
which  the  mercury-vapour  arc  is  produced.  In  another 
direction  the  employment  of  quartz  lenses  makes  it  possible 
to  take  advantage  of  the  optical  properties  of  ultra-violet 
light,  in  connection  with  microscopy ;  for  the  purpose  of 
constructing  a  perfect  optical  system,  crystalline  quartz 

B  2 


244  GLASS  MANUFACTURE. 

would  be  useless,  since  its  property  of  double  refraction 
would  interfere  hopelessly  with  the  performance  of  the 
lenses.  This  is  now  overcome  by  the  use  of  vitreous  silica 
lenses,  in  the  case  of  the  "  ultra-violet  microscope,"  as 
made  by  Carl  Zeiss,  of  Jena.  So  far,  however,  it  has  only 
been  possible  to  produce  quite  small. pieces  of  vitreous  silica 
sufficiently  free  from  bubbles  to  be  used  for  optical 
purposes.  The  great  difficulty  lies  not  so  much  in  merely 
melting  the  quartz  down  as  in  freeing  it  from  the  air- 
bubbles  enclosed  within  it ;  the  course  usually  adopted  with 
glass,  of  raising  the  temperature  and  allowing  the  bubbles 
to  rise  to  the  surface,  becomes  impossible  in  this  case, 
because  the  silica  itself  begins  to  vapourise  and  even  to  boil 
vigorously  at  temperatures  not  very  far  above  its  melting 
point.  Quite  recently,  however,  two  American  workers 
have  claimed  to  be  able  to  overcome  this  difficulty  by  the 
use  of  both  vacuum  and  high  pressure  applied  at  the  earlier 
and  later  stages  of  the  fusion  process  respectively,  so  that 
it  may  shortly  be  possible  to  produce  vitreous  silica  in  large 
and  perfectly  clear  blocks. 

We  have  already  indicated  that  glass  tubing  and  rod 
form  the  basis  upon  which  the  glass-worker,  with  the  aid 
of  the  blow-pipe  or  '  "  lamp,"  fashions  his  productions, 
which,  of  course,  include  a  great  number  of  scientific 
instruments  and  appliances  used  more  especially  in  the 
field  of  chemistry.  In  another  direction  also  glass  tubing 
serves  as  a  basis  for  a  branch  of  the  glass  industry ;  this 
is  the  manufacture  of  certain  classes  of  glass  beads,  which 
are  formed  by  cutting  up  a  heated  glass  tube  of  suitable 
diameter  and  colour  into  short,  more  or  less  spherical 
sections.  In  some  cases  the  colour  of  the  beads  is  secured 


MISCELLANEOUS   PBODUCTS.  245 

by  using  glass  of  the  desired  tint,  but  in  other  cases  the 
beads  are  made  of  colourless  glass,  and  a  colouring  substance 
is  placed  in  the  interior  of  the  bead. 

Solid  glass  rods  are  also  employed  for  a  variety  of 
purposes ;  their  mode  of  manufacture  is  exactly  analogous 
to  that  of  tubing,  except  that  the  gathering  is  drawn  out 
without  having  first  had  a  hollow  space  produced  at  its 
centre  by  the  blower.  In  its  most  attenuated  form  glass 
rod  becomes  glass  thread  or  fibre;  this  is  produced  by 
drawing  hot  glass  very  rapidly,  the  resulting  thread  being 
wound  on  a  large  wheel.  At  one  time  this  material  found 
considerable  use,  since  it  was  found  possible  to  spin  and 
weave  the  thinnest  glass  fibres  into  fabrics  which  could  be 
used  for  dress  purposes.  It  is  not,  however,  to  be  regretted 
that  this  fashion  has  neither  extended  nor  survived,  since 
it  was  certainly  liable  to  produce  serious  injury  to  health. 
It  is  a  well-known  fact  that  there  are  few  more  injurious  or 
even  dangerous  substances  to  be  inhaled  into  the  human 
throat  and  lungs  than  finely-divided  glass;  glass  fibre, 
moreover,  when  subjected  to  constant  bending  and  wear,  is 
bound  to  undergo  frequent  fracture,  and  the  atmosphere  of 
a  ball-room,  for  example,  in  which  several  such  dresses 
were  worn  would  soon  be  contaminated  with  innumerable 
fine,  sharp  particles  of  glass  which  would  produce  an 
injurious  effect  on  those  inhaling  them.  At  the  present 
time  glass  fibre  is  used  for  little  else  than  the  "  glass  wool  " 
required  for  certain  special  purposes  in  chemical  laboratories. 

Fused  quartz  or  silica  fibres,  of  extreme  tenuity,  but  of 
relatively  very  great  strength,  are  employed  in  many 
scientific  instruments,  where  their  extreme  lightness  and 
perfect  elasticity  and  freedom  from  what  is  known  as 


246  GLASS   MANUFACTURE. 

"  elastic  fatigue  "  renders  them  of  very  great  value.  These 
fibres  are  not  drawn  from  a  mass  of  molten  silica,  as  is 
done  with  glass,  but  are  produced  by  attaching  a  nail  or 
bolt  to  a  small  bead  of  fused  silica  produced  by  the  aid  of 
an  oxygen-fed  blowpipe  ;  the  nail  or  bolt  is  then  suddenly 
shot  away  down  a  long  passage  or  similar  space  by  means 
of  a  cross-bow,  drawing  a  very  fine  fibre  of  silica  with  it ; 
the  most  difficult  part  of  this  operation,  however,  consists 
in  finding  and  handling  the  fibres  thus  produced. 

Artificial  Gems. — The  fact  that  pieces  of  suitably-coloured 
glass  can  be  made  to  show  a  superficial,  but  sometimes 
more  or  less  deceptive,  resemblance  to  precious  stones,  has 
led  to  the  manufacture  of  imitation  jewels  of  all  descriptions. 
The  glass  used  for  this  purpose  is  usually  a  very  dense  flint- 
glass  whose  high  refractive  index  facilitates  the  imitation 
which  is  aimed  at.  The  external  shapes  of  gems  are,  of 
course,  readily  imitated  by  cutting  and  grinding  the  glass, 
while  the  requisite  colours  are  attainable  by  means  of  the 
colouring  materials  described  in  Chapter  XI.  To  a  casual 
observer  the  difference  in  sparkle  and  brilliance  which 
arises  from  the  difference  between  the  refractive  index  of 
the  heavy  flint-glass  (about  1*8)  and  that  of  minerals  (which 
ranges  from  1*7  to  2*2)  is  not  readily  apparent,  but  closer 
examination  will  at  once  reveal  the  difference.  The  deter- 
mination of  the  optical  constants  by  means  of  a  refracto- 
meter  would  at  once  reveal  the  true  character  of  the 
imitation,  but  an  even  readier  test  is  that  of  hardness.  The 
dense  flint-glass  is  naturally  soft,  and  is  readily  scratched 
by  most  of  the  harder  minerals,  while  the  precious  stones, 
more  particularly  garnets,  rubies  and  diamonds,  are  very 
hard.  If  an  attempt  is  made  to  scratch  an  ordinary  sheet 


MISCELLANEOUS  PEODUCTS.  247 

of  window-glass,  it  will  be  found  that  most  real  precious 
stones  will  do  so  readily,  while  flint-glass  imitations  will 
fail  to  make  more  than  a  slight  mark,  which  is  more  smear 
than  scratch.  The  test  by  determining  the  specific  gravity 
is  also  obviously  applicable,  since  the  flint-glass  will  readily 
betray  its  presence  by  its  high  density  (over  4). 

In  quite  a  different  class  from  the  imitation  gems  made 
of  cut  flint-glass  are  the  artificial  gems,  which  in  nature 
and  composition  are  exact  reproductions  of  natuial  gems, 
but  which  have  been  produced  by  artificial  processes.  As 
far  as  the  writer  is  aware  these  are  only  found  in  any  large 
numbers  in  the  case  of  the  ruby,  but  in  that  case,  at  all  events, 
it  is  said  that  the  production  of  the  artificial  crystals  is 
at  least  as  costly  as  the  purchase  of  the  natural  stones. 
There  can,  however,  be  very  little  doubt  that  as  the  pro- 
cesses of  fusion  and  crystallisation  become  better  known 
and  understood,  and  the  chemistry  of  silicate  minerals  is 
developed,  the  artificial  production  of  mineral  crystals  in,  at 
all  events,  moderate  sizes  will  become  increasingly  possible ; 
it  is  even  to  be  hoped  that  their  production  will  be  so  far 
perfected  as  to  place  their  really  valuable  properties  at  the 
service  of  man. 

Chilled  Glass. — In  all  the  processes  of  glass  manufacture 
described  in  the  present  book,  annealing  has  always  played 
an  important  part.  The  glass,  after  it  has  undergone  its  last 
treatment  under  the  influence  of  heat,  is  subjected  to  a 
gradual  cooling  process  with  the  object  of  freeing  it  from 
the  internal  strains  which  it  would  otherwise  retain,  and 
which  would,  ordinarily,  endanger  its  existence  and  inter- 
fere with  its  use.  It  is,  however,  well  known  that  surfaces 
of  glass  subjected  to  such  internal  strains  as  result  in  a 


248  GLASS  MANUFACTUEE. 

compressive  stress  on  the  glass  near  the  surface,  are  less 
liable  to  injury,  and  are  apparently  stronger  than  when 
the  glass  is  annealed  and  the  stresses  are  removed.  On 
the  other  hand,  glass  surfaces  under  tension  are  extremely 
delicate  and  fragile.  In  some  respects,  therefore,  glass 
which  has  not  been  annealed  may  appear  to  be  stronger 
than  the  annealed  product.  The  well-known  case  of  the 
Kupert's  drop  is  an  example  of  this  kind.  Rupert's  drops 
are  produced  by  dropping  molten  glass  into  water  ;  they 
generally  take  the  form  of  a  more  or  less  spherical  body 
having  a  long  tail,  tapering  off  into  a  thread,  attached  to  it. 
Such  a  Rupert's  drop  may  be  struck  with  a  heavy  hammer, 
and  will  safely  resist  a  blow  that  would  splinter  a  similar 
body  made  of  annealed  glass.  If,  however,  the  surface  be 
scratched,  or  the  tip  of  the  tail  be  broken  off,'  the  entire 
"  drop  "  breaks  up,  sometimes  with  a  violent  explosion,  into 
minute  fragments.  Numerous  inventors,  among  whom 
De  la  Bastie  and  Siemens  figure  most  conspicuously,  have 
endeavoured  to  utilise  these  properties  of  chilled  glass,  not 
exactly  by  endeavouring  to  produce  that  extreme  degree  of 
internal  strain  which  is  characteristic  of  the  Rupert's  drop, 
but  by  producing  what  they  describe  as  "  tempered  "  glass, 
in  which  the  internal  strains  have  been  reduced  by  less 
violent  cooling  to  such  an  extent  as  to  retain  some  of  the 
advantages  of  the  hardened,  internally  strained  condition 
while  approximating  more  or  less  to  the  safer  state  of 
annealed  glass.  At  one  time  articles  of  this  kind  were 
frequently  seen  as  curiosities,  such  as  tumblers  that  could 
be  dropped  on  the  floor  without  breaking,  etc.,  but  these 
articles  generally  ended  by  receiving  a  slight  scratch  or 
chip  and  promptly  falling  into  fragments.  As  a  matter  of 


MISCELLANEOUS  PEODUOTS.  249 

fact,  however,  some  tempered  glass  is  actually  manufactured 
by  the  firm  of  Siemens  at  the  present  time  for  special 
purposes.  De  la  Bastie's  process  was  tried  in  England,  and 
some  success  was  claimed  for  it ;  but  it  is  not  in  commercial 
operation  at  the  present  time,  and  never  appears  to  have 
attained  any  great  importance. 

Massive  Glass. — Enthusiasts  for  the  extension  of  the  use 
of  glass  have  endeavoured  to  apply  it  to  a  great  variety  of 
purposes,  including  the  construction  of  buildings  and  the 
paving  of  streets.  In  the  former  case,  which  was  exempli- 
fied at  the  Paris  Exhibition  of  1900,  advantage  was  taken 
of  the  light-transmitting  power  of  the  material,  but 
although  the  buildings  erected  with  large  blocks  of  cast 
glass  were  not  displeasing  in  effect,  this  use  has  not  found 
any  considerable  extension.  For  paving  purposes,  the 
hardness  and  durability  of  glass  are  the  only  useful 
qualities,  and  here  also — although  several  trials  have  been 
made  in  France — no  signs  of  any  considerable  application 
of  the  new  products  are  as  yet  visible.  What  has  been 
said  above  with  reference  to  the  injurious  character  of 
glass  dust  applies,  further,  to  glass  pavements,  since  their 
natural  wear  would  result  in  the  formation  of  considerable 
quantities  of  this  dust.  The  advocates  of  glass  paving,  how- 
ever, suggest  that  the  hardness  of  glass  would  greatly 
reduce  the  actual  amount  of  wear,  and  that  consequently 
the  dust  would  be  reduced  considerably.  This  is  a  matter 
which  prolonged  experience  alone  can  decide,  but  it  does 
not  seem  obvious  that  glass  blocks  should  wear  more 
slowly  than  stone  setts  made  of  good  granite,  for  example. 
On  the  other  hand,  the  glass  blocks  could  probably  be  pro- 
duced more  cheaply,  since  the  labour  of  cutting  to  size 


250  GLASS  MANUFACTURE. 

would  be  obviated  by  casting  the  blocks  to  the  desired 
dimensions. 

Water-glass,  or  silicate  of  soda  or  potash  is  perhaps 
scarcely  to  be  classed  under  the  heading  of  "  Glass  Manu- 
facture "  at  all,  but  it  bears  a  certain  relationship  to  glass 
in  several  ways.  Thus  one  of  the  modes  of  manufacturing 
water-glass  is  by  the  fusion  of  sand  and  alkali  in  tank 
furnaces  somewhat  resembling  those  used  for  glass  pro- 
duction ;  the  fused  silicate,  moreover,  solidifies  as  a  vitreous 
mass,  in  which  respect  it  also  resembles  such  substances  as 
borax,  etc.  The  uses  of  silicate  of  soda  and  potash  are, 
however,  so  far  removed  from  the  field  of  glass-manufacture 
that  we  cannot  enter  into  them  here. 

In  concluding  this  chapter,  we  wish  to  describe  one  more 
product  of  the  glassworks,  and  this  includes  some  of  the 
most  impressive  and  splendid  examples  of  the  glass-maker's 
art.  These  are  the  great  mirrors  and  lenses  by  whose  aid 
our  lighthouses  and  searchlights  send  forth  their  powerful 
beams  of  light.  Although  these  objects  are  called  "  mirrors  " 
and  "lenses,"  since  they  fulfil  the  functions  of  such  optical 
organs,  yet  in  their  nature  and  mode  of  manufacture  they 
are  so  far  removed  from  the  glass  used  for  the  production 
of  other  kinds  of  lenses  that  they  could  not  be  included 
under  the  heading  of  "  optical  glass." 

The  characteristic  feature  in  the  manufacture  of  optical 
glass  is  the  manner  in  which  each  separate  pot  or  melting 
is  allowed  to  cool  down  and  to  break  up  into  irregular 
fragments  which  are  subsequently  moulded  to  the  desired 
shape.  Were  it  attempted  to  manufacture  the  large  glass 
bodies  required  for  lighthouse  purposes  in  this  manner, 
the  cost  would  approximate  to  that  of  the  large  discs  used 


MISCELLANEOUS  PEODUCTS.  251 

for  telescope  objectives,  and  this  would  of  course  be  entirely 
prohibitive.  The  requirements  as  regards  colour,  homo- 
geneity and  freedom  from  other  defects,  which  must  be  met 
in  lighthouse  lenses,  are  further  not  nearly  so  stringent  as 
those  which  are  essential  in  ordinary  optical  work  of  good 
quality.  The  reason  for  this  difference  arises  from  the 
fact  that  lighthouse  lenses  and  searchlight  mirrors  are  used 
merely  to  impart  a  desired  direction  to  a  beam  of  light,  and 
not  for  the  purpose  of  producing  sharply-defined  images ; 
slight  irregularities  in  the  glass  are  therefore  not  of  such 
serious  importance. 

Lighthouse  glass  can  therefore  be  produced  by  rather 
less  elaborate  means  ;  although  every  care  is  taken  to  make 
the  glass  as  perfect  as  possible,  it  is  brought  into  approxi- 
mately the  desired  form  by  casting  tbe  molten  glass  in  iron 
moulds  of  the  proper  shape.  When  removed  from  these 
moulds  and  annealed,  the  glass  is  fixed  on  large  revolving 
tables  and  ground  and  polished  to  the  final  shape  of  lenses 
and  annular  lens-segments  as  required  for  the  various 
types  of  Fresnel  lighthouse  lenses.  In  this  way  complete 
rings,  forming  annular  lenses,  are  produced  up  to  48  inches 
diameter.  Kings  of  larger  size  are  usually  built  up  of  a 
number  of  segments,  and  these  built-up  rings  sometimes 
have  a  radius  as  large  as  7  feet.  For  the  majority  of 
lighthouse  lenses,  ifc  should  be  added,  a  hard  soda-lime 
glass  having  a  refractive  index  of  1*50  to  1*52  is  used,  but 
for  special  purposes  a  dense  flint-glass  having  a  refractive 
index  of  1'63  is  employed. 

Mirrors  for  searchlight  purposes  are  of  very  varied  forms 
and  sizes,  the  shape  depending  largely  upon  the  particular 
form  of  beam  which  they  are  designed  to  project.  For 


252  GLASS  MANUFACTURE. 

many  purposes  a  parabolic  form  is  required,  while  in 
others,  where  a  flat,  fan-shaped  beam  is  to  be  produced,  a 
form  having  an  elliptical  section  in  a  horizontal  plane  and 
a  parabolic  section  in  the  vertical  plane  is  required.  In 
most  cases  these  mirrors  are  produced  by  bending  plates  of 
glass,  previously  raised  to  the  necessary  degree  of  heat, 
over  suitably  shaped  moulds,  the  surface  being  subsequently 
re-polished  to  remove  any  roughness  resulting  from  the 
bending  process.  Another  type  of  mirrors  is  that  known 
as  "  Mangin,"  which  has  two  spherical  surfaces  placed 
eccentrically  in  such  a  way  that  the  centre  of  the  mirror 
is  considerably  thinner  than  the  periphery  ;  in  this  type  of 
mirror  the  reflecting  action  of  the  back  surface  is  modified 
by  the  refracting  action  of  the  front  surface,  but  both  are 
spherical,  and  can  therefore  be  accurately  ground  and 
polished  by  the  usual  mechanical  means.  Such  mirrors 
are  manufactured  of  single  pieces  of  glass  up  to  6  feet  in 
diameter. 


APPENDIX 


BIBLIOGEAPHY. 

THE  existing  literature  of  glass  manufacture  is  so  limited  that  a 
complete  bibliography  could  almost  be  given  on  a  single  page  ;  in  the 
English  language,  in  particular,  there  are  exceedingly  few  books  and 
papers  on  the  subject.  The  French  and  German  literature  of  the 
subject  is  a  little  more  extensive.  In  giving  a  list  of  the  works,  and 
more  particularly  in  referring  to  those  which  he  has  consulted  in  the 
preparation  of  the  present  volume,  the  author  thinks  it  will  be  an 
advantage  to  indicate  their  scope,  and,  to  some  extent,  what  he 
believes  to  be  their  value,  in  order  to  save  the  student  the  trouble  of 
seeking  out  comparatively  inaccessible  works  only  to  find  that  they 
contain  little  that  is  of  value  for  his  purpose. 

English  Boohs  and  Papers  on  Glass  Manufacture. 

The  Principles  of  Glass  Making  (George  Bell  &  Sons).  By 
Powell  &  Chance.  An  elementary  book  giving  a  clear  and  concise 
account  of  the  older  processes,  more  especially  in  connection  with  flint 
and  plate-glass. 

Glass.  Articles  in  9th  Edition  of  Encyclopaedia  Britannica.  A 
detailed  account  of  processes,  more  or  less  covering  the  entire  subject, 
but  the  processes  described  are  mostly  obsolete  at  the  present  time. 

Glass.  Article  in  Supplement  to  9th  Edition  of  Encyclopaedia 
Britannica.  By  Harry  J.  Powell.  A  brief  summary  of  more  recent 
developments.  Particularly  valuable  in  reference  to  artistic  English 
flint-glass. 

Jena  Glass.     By  Hovestadt,  translated  by  J.  D.  and  A.  Everett. 


254  APPENDIX. 

Con  bains  a  full  account  of  the  scientific  work  on  glass  and  its  practical 
application,  done  in  connection  with  the  Jena  Works  of  Schott. 
Particularly  interesting  in  connection  with  the  subjects  of  Chapters  I., 
II.,  XII.,  and  XIII.  As  the  title  indicates,  the  book  is  written  from 
the  Jena  point  of  view,  and  scarcely  does  justice  to  work  done 
elsewhere.  The  book  has  gained  considerably  at  the  hands  of  the 
translators. 

Some  Properties  of  Glass.  By  W.  Rosenhain.  (Transactions  of 
the  Optical  Society  of  London,  1903.)  Gives  a  brief  account  of  the 
properties  of  glass  as  affecting  its  optical  uses. 

Possible  Directions  of  Progress  in  Optical  Glass.  By  W.  Rosenhain. 
(Proceedings  of  the  Optical  Convention,  London,  1905.)  Has  been 
referred  to  in  the  text  of  this  book  (Chapter  XII.). 

Catalogue  of  the  Optical  Convention  Exhibition,  London,  1905. 
Contains  historical  and  general  notices  of  optical  and  lighthouse  glass, 
glass-working  machinery,  etc. 

Glass  for  Optical  Instruments.  By  E.  T.  Glazebrook.  (Cantor 
Lectures  to  the  Society  of  Arts.)  Gives  an  account  of  modern  optical 
glass  manufacture. 

Old  English  Glasses.  By  Albert  Hartshorne.  Gives  an  account  of 
the  history  of  glass-making  in  England. 

The  Methods  of  Glass  Blowing.  By  W.  Shenstone.  Describes 
the  manipulation  of  glass-blowing  for  experimental  purposes,  i.e., 
lamp  work. 

French  Books  on  Glass  Manufacture. 

Guide  du  Verrier.  By  G.  Bontemps.  A  classical  work  by  one  of 
the  greatest  experts  of  his  day.  Much  of  the  contents  of  the  book 
is,  however,  entirely  out  of  date  at  the  present  time.  The  book  is 
interesting  as  being  the  work  of  the  man  who  introduced  optical  glass 
manufacture  into  England. 

Verres  et  Emaux.  By  L.  Comgnal.  Chiefly  of  interest  in  connec- 
tion with  the  subjects  of  Chapter  VIII. 

Le  Verre  et  le  Crystal.  By  J.  Henrivaux.  (P.  Vicq  Dunod  et  Cie., 
Paris.)  A  lengthy  book  profusely  illustrated  and  giving  a  great  wealth 
of  detailed  information.  The  writer  was  for  some  time  the  general 
manager  of  one  of  the  largest  plate-glass  manufactories  in  Europe ; 
his  account  of  plate-glass  manufacture  is,  therefore,  especially  valuable. 
Much  space  in  this  book  is  devoted  to  historical  and  aesthetical 
matter. 


APPENDIX.  255 

La  Verrerie  au  XXieine  Siecle.  By  J.  Henrivaux.  (Paris,  K. 
Bernard  et  Cie.,  1903.)  Practically  a  supplement  to  the  preceding  ; 
some  of  the  processes  and  products  described  are,  however,  not  of  a 
practical  nature.  Chiefly  valuable  for  recent  developments  in  plate- 
glass  and  bottle-glass  manufacture. 

German  Books  on  Glass  Manufacture. 

Die  Glasfabrikation.  By  E.  Gerner.  (A.  Hartleben's  Verlag, 
Vienna  and  Leipzig,  1897.)  A  concise  and  clear  account  of  most  of 
the  more  important  processes  of  glass  manufacture.  Very  practical 
in  character.  The  information  given  appears  to  be  reliable,  although 
far  from  complete. 

Die  Herstellung  Grosser  Glaskoerper  and  Die  Bearbeitung  Grosser 
Glaskoerper.  By  C.  Wetzel.  (Hartleben's  Verlag,  Vienna  and 
Leipzig,  1900  and  1901  respectively.)  Describes  numerous  special 
processes  and  appliances  devised  for  use  in  connection  with  large 
glass  objects.  Some  of  these  descriptions,  however,  appear  to  be 
little  more  than  transcripts  from  patent  specifications. 

Glasfabriken  und  Hohlglasfabrikation.  By  E.  Dralle.  (Leipzig, 
Baumgaertner,  1886.)  Looked  upon  as  a  classic  in  Germany.  Gives 
detailed  plans  and  drawings  of  entire  bottle-works,  including  furnaces 
and  all  accessories.  Deals  principally  with  bottle  manufacture. 

Die  Glasfabrikation.  By  Dr.  E.  Tscheuschner.  (Weimar,  B.  H. 
Voigt,  1888.)  A  full  detailed  account  of  all  processes  known  at  the 
time.  The  rapid  progress  of  modern  practice  has,  however,  already 
rendered  this  book  to  some  extent  obsolete. 

Jenaer  Glas.  By  Hovestadt.  Already  referred  to  in  respect  of  the 
English  translation. 

Der  Sprechsaal.  (Schmidt,  Weimar.)  A  trade  journal  devoted  to 
the  discussion  of  technical  matters  relating  to  the  glass  and  ceramic 
industries.  Occasionally  contains  articles  and  abstracts  of  technical 
or  scientific  interest  iri  connection  with  glass  manufacture. 

In  addition  to  the  books  and  papers  named  in  the  above  list,  a 
great  number  of  scientific  papers,  notes,  etc.,  are  to  be  found  scattered 
throughout  the  technical  and  scientific  publications  of  the  world ; 
those  that  have  proved  of  real  interest  and  importance  have,  however, 
left  their  mark  on  the  industry,  and  will  be  found  described  or  referred 
to  in  connection  with  the  various  branches  of  manufacture  described 
n  the  present  volume  or  in  the  books  named  above. 


INDEX 


A. 

ABBE,  8,  10,  210,  213 
Absorption  of  light  in  glass,  82, 

179 
Acid,  action  of,  on  glass,  11 

boric,  action   of,  on   glass, 

11,  186 
carbonic,  action  of,  on  glass, 

12 
hydrofluoric,  action  of,  on 

glass,  12 
phosphoric,    action   of,    on 

glass,  11 

Air,  compressed,  91,  105,  117 
Alkali  chlorides,  use  of,  in  glass 

manufacture,  41 
content     of     hygroscopic 

glass,  6 
metals,  184 
nitrates,  44,  78 
sources  of,  40 
Alkaline    liquids,    action    of,    on 

glass,  11 

Aluminium,  51,  186 
Ammonia  soda,  41 
Anastigmatic  photographic  lenses, 

213 

Ancient  windows,  colours  of,  16, 
202 

G.3I. 


Annealing  bottles,  103 
kiln,  103 

for  optical  glass, 

235 
for    plate    glass, 

135 
for    rolled     plate 

glass,  127 

Anthracite  coal,  42,  53,  79 
Antimony,  188 
Apochromatic  objectives,  213 
Arsenic,  52,  78,  105,  117,  188 
Artificial  gems,  246 
Auerbach,  22 
Aventurine,  185 


B. 


BACTERIA,  action  of,  on  glass,  13 
Barium  compounds,  47,  186 

crown  glass,  212 

glass,  7 
Barytes,  48 
Bases  other  than  alkalies,  sources 

of,  45 

Beads,  244 

Behaviour,  chemical,  of  glass,  6 
Bending  plate  glass,  144 
Bevelling,  145 
Black  ash,  41 


258 


INDEX. 


Blisters  in  sheet  glass,  160,  168 
Blocks,  fire-clay,  58 

tank,  59 

Blower,  sheet  glass,  158 
Blower's  chair,  111 
Blowing  glass,  89 

holes,  91,  161,  189 
sheet  glass,  161 
Blown  glass,  decoration  of,  114 

plate  glass,  171 
Bohemian  glass,  109,  240 
]  foiling  up,  81 
Bottles,  annealing  of,  103 

blowing,     improvements 

in,  99 

machines,  100 
colour  of,  96 
manufacture,  furnace  for, 

97 

moulds  for  blowing,  98 
production  of,  by  hand, 

98 

raw  materials  for,  95 
strength  of,  18 
Boric  acid,  11 
Boron,  186 

Boro-silicate  crown,  212 
Boucher's       bottle  -  blowing 

machine,  101 
Bricks,  fire-clay,  58 

silica,  60 
Bubbles  in  optical  glass,  230 

removal  of,  81 
Burning,  pots,  58 

C. 

CADMIUM,  186 
Calcium  carbonate,  46 

oxide,  45,  186 

sulphate,  47 
Carbon,  53,  79,  186 


Carbonate  of  soda,  41 

Carbonic  acid,  action  of,  on  glass, 

12 

Carboys,  blowing  of,  104 
Casting  plate  glass,  132 
Chair,  glass-blower's,  111 
Chalk,  46 
Chamotte,  57 
Chance,  211 
Charcoal,  42,  53,  79 
Charging  furnaces,  75 
Chemical  behaviour  of  glass,  6 

composition  of  glass,  5 

of   optical 

glass,  217 

reactions  during  fusion, 

76 

Chilled  glass,  247 
Chimneys,  gaslight,  23 

lamp,  238 
Chromium,    colouring    effect    of, 

190 

Cleaning  of  lenses,  220 
Coal,  anthracite,  42,  53,  79 
Cobalt,  colouring  effect  of,  197 
Coke,  42,  53,  79 
Colour  of  ancient  windows,  16 
glass,  32 

theory  of,  181 
optical  glass,  208 
sheet  glass,  167 
Coloured  blown  glass,  113 
glass,  178 

technical  uses  of, 

203 

Combustion  tubing,  7,  241 
Compressed  air  for  glass  blowing, 

91,  105,  117 
Conductivity,  electrical,  of  glass, 

30 

thermal,    of   glass, 
24,  29 


INDEX. 


259 


Copper,  colouring  effect  of,  184 

ruby,  184,  188,  198 
Corrosion  of  glass,  11 
Covered  pots,  56,  109 
Crown,  boro-silicate,  219 

glass,  175,  211 

hard,  212,  219 

soft,  212 

telescope,  215 
Crowns,  furnace,  60 
Crucibles,  manufacture  of,  56 

for  glass  melting,  54 
Crushing  strength  of  glass,  19 
Cryolite,  52 

Crystallisation  of  glass,  3 
Crystals,  mineral,  218 
Gullet,  74 

for  optical  glass,  224 
Cutting  rolled  plate  glass,  128 
Cylinders,  sheet  glass,  161,  171 

D. 

DECOLONISATION    of    glass,    52, 

188,  190,  193,  197 
Decoration  of  blown  glass,  114 
Defects  in  rolled  plate  glass,  129 

sheet  glass,  166 
Definition  of  glass,  1 
De  la  Bastie,  248 
Devitrification,  3,  11 
Diamond,  refractive  index  of,  216 
Dimming  of  glass  surfaces,  12 
Dinas  bricks,  61 
Dipping  of  sheet  glass,  166 
Dispersion  of  optical  glass,  209 

partial,  214 
Double  refraction  in  optical  glass, 

221 

rolling  machine,  130 
Drawing  tubes,  239 
Ductility  of  glass,  20 


Durability  of  glass,  tests  for,  14 
Dust,  action  of,  on  lenses,  220 
glass,  245 

E. 

ELASTICITY  of  glass,  20,  24 
Electrical  properties  of  glass,  29 
Epinal,  39 
Etching  of  glass,  12 
Expansion,  co-efficient  of  thermal, 
24,  25 


F. 


FELSPAR,  40,  44 
Fibres,  glass,  245 
silica,  245 

Figured  rolled  plate  glass,  87, 130 
cutting 
of,  131 

Finger-marks  on  lenses,  219 
Fining  of  glass,  81 

optical  glass,  229 
Fire-clay,  action  of,  on  glass,  6 
for  pots,  55 
wetting  up,  57 
Fire-polish,  117 
Flashed  glass,  25,  199 
Flint,  40 

boro-silicate,  212 
dense,  212,  246 
densest,  212 
extra  dense,  212 
glass,  7,  49,  78,  108,  211 
light,  212 
soda,  212 
telescope,  215 

Flu,orite,  refractive  index  of,  216 
Fontainebleau,  38 
Founding  of  optical  glass,  227 
Fourcault  process,  174 


260 


INDEX. 


Fresnel,  251 
Furnace  crowns,  60 

gas,  63 
Furnaces  for  bottle  manufacture, 

97 

glass  melting,  54,  62 
optical  glass,  225 
plate  glass,  133 
rolled     plate    glass, 

122 

sheet  glass,  151, 170 
ports,  67 

recuperative,  66,  156 
regenerative,  66,  155 
tank,  59,  69 

economy  of,  72 
Fusion,  process  of,  73 

temperature  of  glass,  5 
Freezing  of  glass,  2 

G. 

GASLIGHT,  chimneys  for,  23 

Gas  producers,  62,  64 

Gatherer,  158 

Gathering  of  glass,  85,  88,  158 

Gauge  tubes,  10,  18,  23,  26 

Gems,  artificial,  246 

Ghosts,  photographic,  16 

Glauber's  salt,  43 

Gold,  colouring  effect  of,  185 

Grinding  plate  glass,  137 

Gypsum,  47 

H. 

HARDENED  glass,  20 
Hardness  of  glass,  21 

tests  for,  22 
Heavy  spar,  48 
Henrivaux,  19 
Hertz,  22 


Hock-bottle  colour,  195 
Hohenbocka,  38 
Hollow  glassware,  108 
Horseshoe  flame,  69 
Hydrofluoric  acid,  action  of,  on 

glass,  12 
Hygroscopic  glass,  alkali  content 

of,  6 


r. 


INDENTATION  modulus,  22 

Index,  refractive,  216 

Insulating  properties  of  glass,  29 

Iron,  96 

colouring  effect  of,  196 
oxidation  of,  in  glass,  195 

Irregularities  caused  by  rolling,  86 


J. 


JENA,  7,  10,  14,  26,  29,  203,  210, 
213,  241 


KELP,  40 
Kowalski,  19 


LABORATORY  ware,  10,  23 
Ladling  glass,  85 

rolled  plate  glass,  124 
Lagre,  166 

Lamp-chimneys,  110,  238 
Lamp-work,  240,  244 
Large  vessels,  production  of,  105 
Lead,  49,  183,  188 
Lear  for  rolled  plate  glass,  127 

sheet  glass,  165 
Leighton,  39 


INDEX. 


261 


Lenses,  cleaning  of,  220 

finger-marks  on,  220 
pressing  small,  94 

Light,  action  of,  on  glass,  15 

Lighthouse  glass,  178.  250 

Lime,  slaked,  45 

Lime-stone,  46 

Limited  range  of  vitreous  bodies,  4 

Lippe,  38 

Lynn,  39 


M. 

MACHINES,  bevelling,  145 

double  rolling,  130 

grinding,  139 

polishing,  141 
Magnesia,  48,  186 
Manganese,  15,  52,  80 
Mangin  mirrors,  252 
Marver,  111 
Massive  glass,  249 
Mechanical  properties  of  glass,  18 
Metal,  attachment  of,  to  glass,  26 
Minerals,  crystalline,  217 
Mirrors,  145 

searchlight,  251 
Mixing  of  materials,  73 
Moulds  for  glass-blowing,  90,  110, 
116 

pressed  glass,  119 
Muffled  glass,  172 
Muranese  glass,  123 


N. 

NICKEL,  96 

.        colouring  effect  of,  197 

steel,  27,  148 
Nitrates,  alkali,  44,  78 


O. 


OBJECTIVES,  apochromatic,  213 
telescope,  213 

Opal  glass,  45,  52,  186 

Opaque  plate  glass,  146 

Open  pots,  56 

Optical  glass,  annealing,  235 

chemical    composi- 
tion of,  217 
cooling  of,  233 
cost  of,  237 
fining,  229 
founding,  227 
furnaces  for,  225 
hardness  of,  220 
moulding,  235 
pressing,  93 
range  of,  216 
raw   materials   for, 

223 

sorting,  235 
stability  of,  219 
strain  in,  221 
stirring,  231 
yield  of,  236 
properties  of  glass,  205 


1'. 


PAINTING  on  glass,  201 
Parason,  102 
Patent  plate  glass,  171 
Paving  stones,  glass,  249 
Pearl  ash,  43 
Phosphoric  acid,  11 
Phosphorus,  188 
Photographic  ghosts,  16 

lenses,    anastigma- 

tic,  213 
colour    of, 

209 


262 


INDEX. 


Pipe,  glass-maker's,  89 

sheet-blower's,  158 
warmer,  158 

Plate  glass,  annealing  kiln  for,  135 
bending  of,  144 
blown,  171 
casting,  132 
colour  of,  33 
figured  rolled,  87 
flatness  of,  134 
furnaces  for,  133 
grinding       machines, 

139 

of,  137 
mirrors,  145 
opaque,  146 
polishing     machines, 

141 

of,  137 

raw  materials  for,  132 
rolled,  86,  123 
silvering,  146 
sizes  of,  143 
strength  of,  15 
striae  in,  143 
wired,  27,  147 
Platinum,  27 
Polishing,  theory  of,  141 
Pontil,  98,  176,  239 
Potash,  43 

Potato,  use  of,  in  glass  melting,  81 
Ports,  furnace,  67 
Pots,  burning  of,  58 
covered,  56 
drying  of,  58 
for  flint  glass,  109 

optical  glass,  226 
manufacture  of,  56 
open,  56 

Pouring  of  glass,  85,  87 
Pressed  glass,  92,  118 

composition  of,  120 


Presses  for  glass,  119 
Proofs,  82,  231 
Purity  of  materials,  36 


QUARTZ,  40 


Q. 


E. 


RANGE,     limited,      of      vitreous 

bodies,  4 

Recuperative  furnaces,  66,  156 
Red  lead,  49 
Refraction,     double,     in     optical 

glass,  221 
of    light   in    optical 

glass,  209 

Refractive  index,  216 
Regenerative  furnace,  66,  155 
Reichsanstalt,  10 
Resistance    to    crystallisation   of 

glass,  4 

Rings  for  lighthouse  lenses,  251 
Rod,  glass,  245 
Rolled  plate  glass,  86,  123 

annealing,  127 
cutting,  128 
defects  of,  129 
figured,  130 
furnaces,  123 
ladling,  124 
raw    materials 

for,  124 
rolling,  126 
sorting,  129 
surface  of,  122 
Rolling  of  glass,  86 
Rubies,  artificial,  247 
Ruby,  copper,  184,  188,  198 
flashed,  184 
gold,  185 
Rupert's  drops,  248 


INDEX. 


263 


S. 


SALT-CAKE,  37,  42,  79,  189 

Sand,  38 

Sandstone,  39 

Schott,  8,  19,  203,  213,  241 

Scratches  on  sheet  glass,  169 

Searchlights,  "250 

Seed  in  sheet  glass,  167 

Selenium,    colouring     effect     of, 
190 

Sheet  glass,  70 

blisters  in,  160,  168 
blowing,  161 
colour  of,  33,  167 
compared  with  plate, 

149 

cylinders,  161,  171 
defects  of,  166 
dipping.  166 
flattening,  165 
furnaces,  151,  170 
lear,  165 

mechanical     produc- 
tion of,  173 
raw     materials     for, 

150 

sorting,  166 
splitting,  164 
strength  of,  18 

Siedentopf,  182 

Siege  blocks,  59 

Siemens,  248 

Sievert,  92,  105,  117,  172 

processes,  105,  117 

Signal  glasses,  203 

Silica  bricks,  61 

glass,  5,  26,  241 
sources  of,  37 

Silicon,  colouring  effect  of,  187 

Silver,  colouring  effect  of,  185 

Silvering  plate  glass,  146 


Sizes  of  plate  glass,  142 
Soda  ash,  41 

carbonate,  41 
sulphate,  37,  42,  79 
sulphide,  80 
sulphite,  79 

Solidification  of  glass,  1 
Solutions,  analogy  of,  with  glass, 

206 

Sorting  rolled  plate  glass,  129 
Specific  heat  of  glass,  25.  29 

inductive      capacity      of 

glass,  29 

Stains,  coloured,  200 
Stassfurth,  44 
Stones  in  rolled  plate  glass,  129 

sheet  glass,  167 
Storage  of  materials,  37 
Strain  in  optical  glass,  221 
Strength  of  glass,  19 
Striae  in  coloured  glass,  203 

optical  glass,  206,  227 
plate  glass,  143 
testing  apparatus,  207 
String  in  sheet  glass,  168 
Strontium,  86 
Structure  of  glass,  1 
Sulphur,      colouring      effect      of. 

189 
Surfaces,  chemical   behaviour  of 

glass,  8,  10 
S/igmondi,  182 


T. 


TABLE,  rolling,  126 
Tank  blocks,  59 

furnaces,  59,  69 

economy  of,  72 
for    sheet    glass, 
152 


264 


INDEX. 


Telescope  objectives,  213 
Temperature  of  fusion  of  glass, 

5 

Tempered  glass,  20,  248 
Tensile  strength  of  glass,  19 
Thallium,  183,  188 
Theory  of  colours  in  glass,  181 

polishing,  141 

Thermal  endurance  of  glass,  23 
properties  of  glass,  23 
Thermometer  glass,  7,  8,  28 
Tin,  colouring  effect  of,  187 
Tonnelot,  7 
Transparency  of  glass,  31 

optical  glass,  208 
Trautwine,  19 
Tubing,  238 

combustion,  7 
drawing  of,  239 
Tumblers,  111 


U. 


ULTKA-VIOLET  microscope,  243 


V. 

VANADIUM,  colouring  effect  of,  189 
Veins  in  optical  glass,  206,  227 

w. 

WATEE,  action  of,  011  glass,  10 

glass,  250 

Wetting  up  clay,  57 
Winkelmann,  19 
Wired  plate  glass,  27,  147 
Witherite,  48 
Wool,  glass,  245 

Y. 

YOUNG'S  modulus,  20 

Z, 

ZAFFRE,  197 

Zeiss,  213,  244 

Zinc,  colouring  effect  of,  49,  186 

Zschimmer,  14 


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